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/types/admin.types.ts

@@ -14,6 +14,15 @@ export interface PingResult {
   status: 'up' | 'down';
   latencyMs: number;
   error?: string;
+  checked_at?: string;
+}
+
+export interface PingHistoryEntry {
+  id: number;
+  service_id: string;
+  status: string;
+  response_time_ms: number | null;
+  checked_at: string;
 }
 
 export interface StatusSummary {

frontend/src/types/config.types.ts

@@ -1,8 +1,15 @@
+export interface LinkCollection {
+  id: string;
+  name: string;
+  links: Array<{ name: string; url: string }>;
+}
+
 export interface ExternalLinks {
   nextcloud?: string;
   bookstack?: string;
   vikunja?: string;
   customLinks?: Array<{ name: string; url: string }>;
+  linkCollections?: LinkCollection[];
 }
 
 export interface ServiceModeStatus {