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/admin/ServiceModeTab.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
 import {
   Box, Card, CardContent, Typography, Switch, FormControlLabel,
   TextField, Button, Alert, CircularProgress,
@@ -20,17 +20,19 @@ export default function ServiceModeTab() {
   const currentValue = setting?.value ?? { active: false, message: '' };
   const [active, setActive] = useState<boolean>(currentValue.active ?? false);
   const [message, setMessage] = useState<string>(currentValue.message ?? '');
+  const [endsAt, setEndsAt] = useState<string>('');
 
   // Sync state when data loads
-  useState(() => {
+  useEffect(() => {
     if (setting?.value) {
       setActive(setting.value.active ?? false);
       setMessage(setting.value.message ?? '');
+      setEndsAt(setting.value.ends_at ?? '');
     }
-  });
+  }, [setting]);
 
   const mutation = useMutation({
-    mutationFn: (value: { active: boolean; message: string }) =>
+    mutationFn: (value: { active: boolean; message: string; ends_at?: string | null }) =>
       settingsApi.update('service_mode', value),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['service-mode'] });
@@ -41,7 +43,7 @@ export default function ServiceModeTab() {
   });
 
   const handleSave = () => {
-    mutation.mutate({ active, message });
+    mutation.mutate({ active, message, ends_at: endsAt || null });
   };
 
   if (isLoading) {
@@ -63,6 +65,12 @@ export default function ServiceModeTab() {
             </Alert>
           )}
 
+          {active && endsAt && (
+            <Alert severity="info" sx={{ mb: 2 }}>
+              Wartungsmodus endet automatisch am {new Date(endsAt).toLocaleString('de-DE')}.
+            </Alert>
+          )}
+
           <FormControlLabel
             control={
               <Switch
@@ -87,6 +95,17 @@ export default function ServiceModeTab() {
             helperText="Diese Nachricht sehen Benutzer auf der Wartungsseite."
           />
 
+          <TextField
+            fullWidth
+            label="Automatisch deaktivieren am"
+            type="datetime-local"
+            value={endsAt}
+            onChange={(e) => setEndsAt(e.target.value)}
+            InputLabelProps={{ shrink: true }}
+            helperText="Optional: Wartungsmodus wird automatisch zu diesem Zeitpunkt deaktiviert."
+            sx={{ mb: 3, '& input': { color: 'text.primary' } }}
+          />
+
           <Button
             variant="contained"
             color={active ? 'error' : 'primary'}