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/components/chat/ChatPanel.tsx

@@ -29,7 +29,9 @@ const ChatPanelInner: React.FC = () => {
         elevation={2}
         sx={{
           width: COLLAPSED_WIDTH,
-          height: '100%',
+          height: '100vh',
+          position: 'sticky',
+          top: 0,
           display: { xs: 'none', sm: 'flex' },
           flexDirection: 'column',
           alignItems: 'center',
@@ -90,12 +92,12 @@ const ChatPanelInner: React.FC = () => {
       elevation={2}
       sx={{
         width: { xs: '100vw', sm: EXPANDED_WIDTH },
-        position: { xs: 'fixed', sm: 'relative' },
-        top: { xs: 0, sm: 'auto' },
+        position: { xs: 'fixed', sm: 'sticky' },
+        top: { xs: 0, sm: 0 },
         right: { xs: 0, sm: 'auto' },
         bottom: { xs: 0, sm: 'auto' },
         zIndex: { xs: (theme) => theme.zIndex.drawer + 2, sm: 'auto' },
-        height: '100%',
+        height: { xs: '100vh', sm: '100vh' },
         display: 'flex',
         flexDirection: 'column',
         flexShrink: 0,