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/config.controller.ts

@@ -19,13 +19,14 @@ class ConfigController {
     if (environment.bookstack.url) envLinks.bookstack = environment.bookstack.url;
     if (environment.vikunja.url) envLinks.vikunja = environment.vikunja.url;
 
-    const customLinks = await settingsService.getExternalLinks();
+    const linkCollections = await settingsService.getExternalLinks();
 
     res.status(200).json({
       success: true,
       data: {
         ...envLinks,
-        customLinks,
+        customLinks: linkCollections.flatMap(c => c.links),
+        linkCollections,
       },
     });
   }