feat: add notification center with bell dropdown, full page, and workspace scoping (Story 3.3)

Enhance NotificationDropdown with type-specific icons, French description builder,
click-to-navigate with mark-as-read, and "Voir toutes les notifications" link.
Add full notifications page at /notifications with pagination (25/page), individual
mark-as-read, and empty state. Includes code review fixes: workspace-scoped unread
count and dropdown items, race condition fix (mark-as-read before navigate),
efficient markAllAsRead via direct update, deleted declaration URL handling,
and per-workspace cache keys. 7 new feature tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/Http/Controllers/FolderMentionController.php

@@ -52,7 +52,7 @@ class FolderMentionController extends Controller
             $validated['message'],
         ));
 
-        Cache::forget("user:{$targetUser->id}:unread_notifications");
+        Cache::forget("user:{$targetUser->id}:workspace:{$workspace->id}:unread_notifications");
 
         return back()->with('flash', ['type' => 'success', 'message' => 'Notification envoyée.']);
     }