Extract shared helpers from artists.js to shared-helpers.js

Part C of the deferred unification cleanup. The Artists page is no
longer in the sidebar, but its JS file can't be deleted yet because
it houses ~20 general-purpose helpers that other modules depend on
(escapeHtml used in 229 places, service-status polling, image-colour
extraction, download-bubble infrastructure, discography completion
checking, enrichment card rendering).

Moved all non-page-specific code from artists.js into the new
webui/static/shared-helpers.js — pure copy/paste, zero logic change.
Two contiguous blocks extracted:

  Block A (lines 1097..1398 of original artists.js): discography
    completion suite — checkDiscographyCompletion, handleStreaming-
    CompletionUpdate, cacheCompletionData, updateAlbumCompletion-
    Overlay, getCompletionStatusText, setAlbumDownloadedStatus,
    setAlbumDownloadingStatus.

  Block B (lines 2206..EOF of original artists.js): download-bubble
    infrastructure (artist + search + Beatport clusters with their
    snapshot/hydrate/modal/monitor helpers), openDownloadMissingModal-
    ForArtistAlbum, image-colour extractor and dynamic-glow helper,
    escapeHtml, service-status polling, renderEnrichmentCards.

Function declarations in a plain <script> tag are auto-global, so all
existing callers continue to resolve without any import/export
changes. Load order in index.html: shared-helpers.js loads right
after core.js (which defines the artistDownloadBubbles / search-
DownloadBubbles / beatportDownloadBubbles globals these helpers use).

Stats:
  artists.js:       4638 → 1903 lines (-2735)
  shared-helpers.js: new, 2762 lines
  No function duplicated between the two files
  All 357 tests pass (3 new from split-integrity parametrization)

What's left in artists.js is purely the Artists page — search UI,
detail view, state switching, watchlist button, discography loading.
All of that is reachable only by typing /artists in the URL bar
since the sidebar entry was retired in Phase 4b. Parts D + E will
delete that remainder and the file itself.

tests/test_script_split_integrity.py

@@ -27,9 +27,11 @@ _ROOT = Path(__file__).resolve().parent.parent
 _STATIC = _ROOT / "webui" / "static"
 _INDEX = _ROOT / "webui" / "index.html"
 
-# The 17 modules that replaced script.js (order matters for first/last checks)
+# The 17 modules that replaced script.js + shared-helpers.js extracted from
+# artists.js (order matters for first/last checks)
 SPLIT_MODULES = [
     "core.js",
+    "shared-helpers.js",
     "media-player.js",
     "settings.js",
     "search.js",

webui/index.html

@@ -7988,6 +7988,7 @@
     <script src="{{ url_for('static', filename='setup-wizard.js') }}"></script>
     <!-- Split modules (was: script.js) — core.js must load first, init.js last -->
     <script src="{{ url_for('static', filename='core.js') }}"></script>
+    <script src="{{ url_for('static', filename='shared-helpers.js') }}"></script>
     <script src="{{ url_for('static', filename='media-player.js') }}"></script>
     <script src="{{ url_for('static', filename='settings.js') }}"></script>
     <script src="{{ url_for('static', filename='search.js') }}"></script>