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/controllers/serviceMonitor.controller.ts

@@ -137,7 +137,7 @@ class ServiceMonitorController {
   async getUsers(_req: Request, res: Response): Promise<void> {
     try {
       const result = await pool.query(
-        `SELECT id, email, name, role, authentik_groups as groups, is_active, last_login_at
+        `SELECT id, email, name, authentik_groups as groups, is_active, last_login_at
          FROM users ORDER BY name`
       );
       res.json({ success: true, data: result.rows });
@@ -147,6 +147,17 @@ class ServiceMonitorController {
     }
   }
 
+  async getPingHistory(req: Request, res: Response): Promise<void> {
+    try {
+      const { serviceId } = req.params;
+      const data = await serviceMonitorService.getPingHistory(serviceId as string);
+      res.json({ success: true, data });
+    } catch (error) {
+      logger.error('Failed to get ping history', { error });
+      res.status(500).json({ success: false, message: 'Failed to get ping history' });
+    }
+  }
+
   async broadcastNotification(req: Request, res: Response): Promise<void> {
     try {
       const { titel, nachricht, schwere, targetGroup } = broadcastSchema.parse(req.body);