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>

frontend/src/services/admin.ts

@@ -1,5 +1,5 @@
 import { api } from './api';
-import type { MonitoredService, PingResult, StatusSummary, SystemHealth, UserOverview, BroadcastPayload } from '../types/admin.types';
+import type { MonitoredService, PingResult, PingHistoryEntry, StatusSummary, SystemHealth, UserOverview, BroadcastPayload } from '../types/admin.types';
 
 interface ApiResponse<T> {
   success: boolean;
@@ -16,4 +16,5 @@ export const adminApi = {
   getSystemHealth: () => api.get<ApiResponse<SystemHealth>>('/api/admin/system/health').then(r => r.data.data),
   getUsers: () => api.get<ApiResponse<UserOverview[]>>('/api/admin/users').then(r => r.data.data),
   broadcast: (data: BroadcastPayload) => api.post<ApiResponse<{ sent: number }>>('/api/admin/notifications/broadcast', data).then(r => r.data.data),
+  getPingHistory: (serviceId: string) => api.get<ApiResponse<PingHistoryEntry[]>>(`/api/admin/services/${serviceId}/ping-history`).then(r => r.data.data),
 };