feat(#251): expose backend error detail in apiFetch errors

Le wrapper apiFetch jetait `API error: 422` sans le body. Désormais lit
detail (string ou liste Pydantic) et le concatène au message :
`422: Neo4j config requires a non-empty 'index_name'`.

Permet aux pages StoreCreatePage / StoreEditPage de surfacer la cause
réelle du refus serveur dans l'errorMessage du form.

frontend/src/shared/api/http.ts

@@ -13,7 +13,31 @@ export async function apiFetch<T = unknown>(url: string, options: FetchOptions =
     ...options,
     headers,
   })
-  if (!response.ok) throw new Error(`API error: ${response.status}`)
+  if (!response.ok) {
+    const detail = await readErrorDetail(response)
+    throw new Error(detail ? `${response.status}: ${detail}` : `API error: ${response.status}`)
+  }
   if (response.status === 204) return null as T
   return response.json() as Promise<T>
 }
+
+async function readErrorDetail(response: Response): Promise<string | null> {
+  try {
+    const contentType = response.headers.get('content-type') ?? ''
+    if (!contentType.includes('application/json')) return null
+    const body = await response.json()
+    if (typeof body?.detail === 'string') return body.detail
+    if (Array.isArray(body?.detail)) {
+      return body.detail
+        .map((e: { loc?: unknown[]; msg?: string }) => {
+          const loc = Array.isArray(e.loc) ? e.loc.join('.') : ''
+          return loc ? `${loc}: ${e.msg ?? ''}` : (e.msg ?? '')
+        })
+        .filter(Boolean)
+        .join('; ')
+    }
+    return null
+  } catch {
+    return null
+  }
+}