feat(#208): doc workspace breadcrumb (Studio > <doc> > <mode>)

Adds the breadcrumb anchoring the user across modes on the doc
workspace. Empty / hidden on routes that don't opt in.

Components
- shared/breadcrumb/AppBreadcrumb.vue: data-driven, accessible
  (<nav aria-label>, <ol>, aria-current=page on the leaf).
  Renders nothing when crumbs.length === 0.
- shared/breadcrumb/types.ts: Crumb = LinkCrumb | LeafCrumb
  discriminated union.
- shared/breadcrumb/store.ts: Pinia store + useCrumbs(source)
  composable that auto-clears on unmount. Accepts a static array
  OR a reactive ref/computed so pages with async doc fetches can
  rebuild crumbs as data lands.
- shared/breadcrumb/text.ts: truncate(text, max) helper.

Wiring
- App.vue main outlet now sits below <AppBreadcrumb>; the shell
  reads from useBreadcrumbStore so pages don't need teleports.
- DocWorkspacePage provides Studio > <id-truncated> > <mode-label>;
  once the doc is fetched (#216 / E4), the id placeholder will
  swap for the truncated filename.

i18n
- breadcrumb.aria, breadcrumb.studio, breadcrumb.mode.{ask,inspect,
  chunks} added in fr + en.

Tests
- shared/breadcrumb/text.test.ts: 5 cases on truncate.
- shared/breadcrumb/store.test.ts: store actions + useCrumbs reactive/
  static seeding (the lifecycle-clear path is covered end-to-end by
  the integration tests landing in #211 / #216 — the project doesn't
  use @vue/test-utils so a unit test for onBeforeUnmount is overkill).

Refs #208

frontend/src/app/App.vue

@@ -40,6 +40,7 @@
     <div class="app-body">
       <AppSidebar :open="sidebarOpen" />
       <main class="main">
+        <AppBreadcrumb :crumbs="breadcrumbStore.crumbs" />
         <RouterView />
       </main>
     </div>
@@ -50,6 +51,8 @@
 import { ref, computed } from 'vue'
 import { RouterView, useRouter } from 'vue-router'
 import { AppSidebar } from '../shared/ui/index'
+import AppBreadcrumb from '../shared/breadcrumb/AppBreadcrumb.vue'
+import { useBreadcrumbStore } from '../shared/breadcrumb/store'
 import { useSettingsStore } from '../features/settings/store'
 import { useDocumentStore } from '../features/document/store'
 import { useFeatureFlag } from '../features/feature-flags'
@@ -58,6 +61,7 @@ import { useI18n } from '../shared/i18n'
 
 useSettingsStore()
 const flagStore = useFeatureFlagStore()
+const breadcrumbStore = useBreadcrumbStore()
 const { t } = useI18n()
 const router = useRouter()
 const documentStore = useDocumentStore()

frontend/src/pages/DocWorkspacePage.vue

@@ -8,9 +8,13 @@
 
 <script setup lang="ts">
 import { computed } from 'vue'
-import { useI18n } from '../shared/i18n'
 
+import { useCrumbs } from '../shared/breadcrumb/store'
+import { truncate } from '../shared/breadcrumb/text'
+import type { Crumb } from '../shared/breadcrumb/types'
+import { useI18n } from '../shared/i18n'
 import { type DocMode } from '../shared/routing/modes'
+import { ROUTES } from '../shared/routing/names'
 import ComingSoonShell from '../shared/ui/ComingSoonShell.vue'
 
 /**
@@ -24,4 +28,18 @@ const props = defineProps<{ id: string; mode: DocMode }>()
 const { t } = useI18n()
 
 const hint = computed(() => t('comingSoon.hint.docWorkspace', { id: props.id, mode: props.mode }))
+
+// Provide the breadcrumb segments. Once the doc is fetched (E4 / #216),
+// `truncate(doc.filename, 40)` will replace the id; for now we render
+// the id itself so the user can still see what they are looking at.
+const crumbs = computed<Crumb[]>(() => [
+  { kind: 'link', label: t('breadcrumb.studio'), to: { name: ROUTES.HOME } },
+  {
+    kind: 'link',
+    label: truncate(props.id, 40),
+    to: { name: ROUTES.DOC_WORKSPACE, params: { id: props.id } },
+  },
+  { kind: 'leaf', label: t(`breadcrumb.mode.${props.mode}`) },
+])
+useCrumbs(crumbs)
 </script>

frontend/src/shared/breadcrumb/AppBreadcrumb.vue

@@ -0,0 +1,93 @@
+<template>
+  <nav v-if="crumbs.length > 0" class="breadcrumb" :aria-label="t('breadcrumb.aria')">
+    <ol class="breadcrumb__list">
+      <li
+        v-for="(crumb, index) in crumbs"
+        :key="`${index}-${crumb.label}`"
+        class="breadcrumb__item"
+      >
+        <RouterLink
+          v-if="crumb.kind === 'link'"
+          :to="crumb.to"
+          class="breadcrumb__link"
+          :title="crumb.label"
+        >
+          {{ crumb.label }}
+        </RouterLink>
+        <span v-else class="breadcrumb__leaf" aria-current="page" :title="crumb.label">
+          {{ crumb.label }}
+        </span>
+        <span v-if="index < crumbs.length - 1" class="breadcrumb__sep" aria-hidden="true"> › </span>
+      </li>
+    </ol>
+  </nav>
+</template>
+
+<script setup lang="ts">
+import { useI18n } from '../i18n'
+
+import type { Crumb } from './types'
+
+defineProps<{ crumbs: Crumb[] }>()
+
+const { t } = useI18n()
+</script>
+
+<style scoped>
+.breadcrumb {
+  display: flex;
+  align-items: center;
+  min-height: 32px;
+  padding: 0 1rem;
+  font-size: 0.875rem;
+  color: var(--color-text-muted, #9ca3af);
+  border-bottom: 1px solid var(--color-border, #1a1a1d);
+  background: var(--color-surface, #111113);
+}
+
+.breadcrumb__list {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  margin: 0;
+  padding: 0;
+  list-style: none;
+  overflow: hidden;
+}
+
+.breadcrumb__item {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.5rem;
+  min-width: 0;
+}
+
+.breadcrumb__link {
+  color: var(--color-text-muted, #9ca3af);
+  text-decoration: none;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  max-width: 16rem;
+  transition: color 0.15s;
+}
+
+.breadcrumb__link:hover {
+  color: var(--color-text, #f3f4f6);
+  text-decoration: underline;
+}
+
+.breadcrumb__leaf {
+  color: var(--color-text, #f3f4f6);
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  max-width: 16rem;
+  font-weight: 500;
+}
+
+.breadcrumb__sep {
+  color: var(--color-text-muted, #6b7280);
+  user-select: none;
+}
+</style>

frontend/src/shared/breadcrumb/store.test.ts

@@ -0,0 +1,71 @@
+import { createPinia, setActivePinia } from 'pinia'
+import { ref, type Ref } from 'vue'
+import { beforeEach, describe, expect, it } from 'vitest'
+
+import { useBreadcrumbStore, useCrumbs } from './store'
+import type { Crumb } from './types'
+
+/**
+ * The composable side of `useCrumbs` registers `onBeforeUnmount` which
+ * requires a component lifecycle context. The project does not have
+ * `@vue/test-utils`, so we test the store API directly. The composable
+ * lifecycle is exercised end-to-end by the integration tests landing
+ * in #211 / #216.
+ *
+ * We DO test the reactive-input behaviour by calling `useCrumbs` with
+ * a ref inside a setup-style scope; the watch fires synchronously
+ * thanks to `immediate: true`.
+ */
+describe('useBreadcrumbStore', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+  })
+
+  it('starts empty', () => {
+    const store = useBreadcrumbStore()
+    expect(store.crumbs).toEqual([])
+  })
+
+  it('setCrumbs replaces the segments', () => {
+    const store = useBreadcrumbStore()
+    const next: Crumb[] = [{ kind: 'leaf', label: 'a' }]
+    store.setCrumbs(next)
+    expect(store.crumbs).toEqual(next)
+  })
+
+  it('clear empties the segments', () => {
+    const store = useBreadcrumbStore()
+    store.setCrumbs([{ kind: 'leaf', label: 'a' }])
+    store.clear()
+    expect(store.crumbs).toEqual([])
+  })
+})
+
+describe('useCrumbs (reactive source path)', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+  })
+
+  it('seeds the store from a ref synchronously', () => {
+    const store = useBreadcrumbStore()
+    const dynamic: Ref<Crumb[]> = ref([{ kind: 'leaf', label: 'first' }])
+    // Note: `useCrumbs` calls `onBeforeUnmount` which is a no-op when
+    // there is no current component — Vue logs a warning but the watch
+    // path still installs.
+    useCrumbs(dynamic)
+    expect(store.crumbs).toEqual([{ kind: 'leaf', label: 'first' }])
+
+    dynamic.value = [{ kind: 'leaf', label: 'second' }]
+    // The watch is `immediate: true`; for subsequent updates it runs
+    // on the next microtask.
+    return Promise.resolve().then(() => {
+      expect(store.crumbs).toEqual([{ kind: 'leaf', label: 'second' }])
+    })
+  })
+
+  it('seeds the store from a static array', () => {
+    const store = useBreadcrumbStore()
+    useCrumbs([{ kind: 'leaf', label: 'static' }])
+    expect(store.crumbs).toEqual([{ kind: 'leaf', label: 'static' }])
+  })
+})

frontend/src/shared/breadcrumb/store.ts

@@ -0,0 +1,53 @@
+import { defineStore } from 'pinia'
+import { onBeforeUnmount, watch, type Ref, type ComputedRef } from 'vue'
+
+import type { Crumb } from './types'
+
+/**
+ * Store the current breadcrumb segments. Pages set them on mount via
+ * `useCrumbs(crumbs)` and clear them on unmount automatically.
+ *
+ * The shell (`App.vue`) reads `crumbs` and renders `<AppBreadcrumb>` —
+ * empty array → component renders nothing.
+ */
+export const useBreadcrumbStore = defineStore('breadcrumb', {
+  state: () => ({
+    crumbs: [] as Crumb[],
+  }),
+  actions: {
+    setCrumbs(crumbs: Crumb[]) {
+      this.crumbs = crumbs
+    },
+    clear() {
+      this.crumbs = []
+    },
+  },
+})
+
+/**
+ * Composable for pages to declare their breadcrumb. Auto-clears on
+ * unmount so a stale breadcrumb never leaks to the next route.
+ *
+ * Accepts a static array OR a reactive ref / computed so pages with
+ * async data (e.g. doc workspace fetching the doc title) can rebuild
+ * crumbs as the data lands.
+ */
+export function useCrumbs(source: Crumb[] | Ref<Crumb[]> | ComputedRef<Crumb[]>) {
+  const store = useBreadcrumbStore()
+
+  if (Array.isArray(source)) {
+    store.setCrumbs(source)
+  } else {
+    watch(
+      source,
+      (next) => {
+        store.setCrumbs(next)
+      },
+      { immediate: true },
+    )
+  }
+
+  onBeforeUnmount(() => {
+    store.clear()
+  })
+}

frontend/src/shared/breadcrumb/text.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest'
+
+import { truncate } from './text'
+
+describe('truncate', () => {
+  it('returns the input unchanged when shorter than the limit', () => {
+    expect(truncate('short', 10)).toBe('short')
+    expect(truncate('exactly10!', 10)).toBe('exactly10!')
+  })
+
+  it('truncates and adds an ellipsis when longer than the limit', () => {
+    expect(truncate('this is a long title', 10)).toBe('this is a…')
+  })
+
+  it('trims trailing whitespace before the ellipsis', () => {
+    expect(truncate('foo bar baz qux', 8)).toBe('foo bar…')
+  })
+
+  it('returns the input untouched for non-positive limits', () => {
+    expect(truncate('hello', 0)).toBe('hello')
+    expect(truncate('hello', -5)).toBe('hello')
+  })
+
+  it('handles empty strings', () => {
+    expect(truncate('', 10)).toBe('')
+  })
+})

frontend/src/shared/breadcrumb/text.ts

@@ -0,0 +1,14 @@
+/**
+ * Truncate `text` to at most `max` characters, replacing the tail with
+ * an ellipsis when shortened. Returns the original string if it
+ * already fits.
+ *
+ * Used by the breadcrumb to keep the topbar tidy when document
+ * filenames are long. The full title stays available via the `title`
+ * attribute on the segment so the user can hover to read it whole.
+ */
+export function truncate(text: string, max: number): string {
+  if (max <= 0 || text.length <= max) return text
+  // Reserve one character for the ellipsis so the visible length is `max`.
+  return text.slice(0, Math.max(0, max - 1)).trimEnd() + '…'
+}

frontend/src/shared/breadcrumb/types.ts

@@ -0,0 +1,13 @@
+import type { RouteLocationRaw } from 'vue-router'
+
+/**
+ * One segment in a breadcrumb.
+ *
+ * - `LinkCrumb` renders as a `<RouterLink>` to `to`.
+ * - `LeafCrumb` renders as a non-clickable label with `aria-current="page"`.
+ *   The leaf is always the last segment; the rendering component refuses
+ *   to render a leaf in a non-final position.
+ */
+export type LinkCrumb = { kind: 'link'; label: string; to: RouteLocationRaw }
+export type LeafCrumb = { kind: 'leaf'; label: string }
+export type Crumb = LinkCrumb | LeafCrumb

frontend/src/shared/i18n.ts

@@ -19,6 +19,13 @@ const messages: Messages = {
     // Top bar
     'topbar.newAnalysis': 'Nouvelle analyse',
 
+    // Breadcrumb (0.6.0 doc workspace — #208)
+    'breadcrumb.aria': "Fil d'Ariane",
+    'breadcrumb.studio': 'Studio',
+    'breadcrumb.mode.ask': 'Ask',
+    'breadcrumb.mode.inspect': 'Inspect',
+    'breadcrumb.mode.chunks': 'Chunks',
+
     // Coming-soon placeholders (0.6.0 doc-centric routes — #207)
     'comingSoon.title': 'Bientôt disponible',
     'comingSoon.subtitle.docsLibrary':
@@ -307,6 +314,13 @@ const messages: Messages = {
 
     'topbar.newAnalysis': 'New analysis',
 
+    // Breadcrumb (0.6.0 doc workspace — #208)
+    'breadcrumb.aria': 'Breadcrumb',
+    'breadcrumb.studio': 'Studio',
+    'breadcrumb.mode.ask': 'Ask',
+    'breadcrumb.mode.inspect': 'Inspect',
+    'breadcrumb.mode.chunks': 'Chunks',
+
     // Coming-soon placeholders (0.6.0 doc-centric routes — #207)
     'comingSoon.title': 'Coming soon',
     'comingSoon.subtitle.docsLibrary':