feat: bug fixes, layout improvements, and new features

Bug fixes:
- Remove non-existent `role` column from admin users SQL query (A1)
- Fix Nextcloud Talk chat API path v4 → v1 for messages/send/read (A2)
- Fix ServiceModeTab sync: useState → useEffect to reflect DB state (A3)
- Guard BookStack book_slug with book_id fallback to avoid broken URLs (A4)

Layout & UI:
- Chat panel: sticky full-height positioning, main content scrolls independently (B1)
- Vehicle booking datetime inputs: explicit text color for dark mode (B2)
- AnnouncementBanner moved into grid with full-width span (B3)

Features:
- Per-user widget visibility preferences stored in users.preferences JSONB (C1)
- Link collections: grouped external links in admin UI and dashboard widget (C2)
- Admin ping history: migration 026, checked_at timestamps, expandable history rows (C4)
- Service mode end date picker with scheduled deactivation display (C5)
- Vikunja startup config logging and configured:false warnings (C7)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/server.ts

@@ -31,6 +31,13 @@ const startServer = async (): Promise<void> => {
         environment: environment.nodeEnv,
         database:    dbConnected ? 'connected' : 'disconnected',
       });
+
+      // Log integration status so operators can see what's configured
+      logger.info('Integration status', {
+        bookstack:  !!environment.bookstack.url,
+        nextcloud:  !!environment.nextcloudUrl,
+        vikunja:    !!environment.vikunja.url,
+      });
     });
 
     // Graceful shutdown handling