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/pages/Settings.tsx

@@ -11,13 +11,41 @@ import {
   Box,
   ToggleButtonGroup,
   ToggleButton,
+  CircularProgress,
 } from '@mui/material';
-import { Settings as SettingsIcon, Notifications, Palette, Language, SettingsBrightness, LightMode, DarkMode } from '@mui/icons-material';
+import { Settings as SettingsIcon, Notifications, Palette, Language, SettingsBrightness, LightMode, DarkMode, Widgets } from '@mui/icons-material';
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import DashboardLayout from '../components/dashboard/DashboardLayout';
 import { useThemeMode } from '../contexts/ThemeContext';
+import { preferencesApi } from '../services/settings';
+import { WIDGETS, WidgetKey } from '../constants/widgets';
 
 function Settings() {
   const { themeMode, setThemeMode } = useThemeMode();
+  const queryClient = useQueryClient();
+
+  const { data: preferences, isLoading: prefsLoading } = useQuery({
+    queryKey: ['user-preferences'],
+    queryFn: preferencesApi.get,
+  });
+
+  const mutation = useMutation({
+    mutationFn: preferencesApi.update,
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['user-preferences'] });
+    },
+  });
+
+  const isWidgetVisible = (key: WidgetKey) => {
+    return preferences?.widgets?.[key] !== false;
+  };
+
+  const toggleWidget = (key: WidgetKey) => {
+    const current = preferences ?? {};
+    const widgets = { ...(current.widgets ?? {}) };
+    widgets[key] = !isWidgetVisible(key);
+    mutation.mutate({ ...current, widgets });
+  };
 
   return (
     <DashboardLayout>
@@ -27,6 +55,40 @@ function Settings() {
         </Typography>
 
         <Grid container spacing={3}>
+          {/* Widget Visibility */}
+          <Grid item xs={12} md={6}>
+            <Card>
+              <CardContent>
+                <Box sx={{ display: 'flex', alignItems: 'center', mb: 2 }}>
+                  <Widgets color="primary" sx={{ mr: 2 }} />
+                  <Typography variant="h6">Dashboard-Widgets</Typography>
+                </Box>
+                <Divider sx={{ mb: 2 }} />
+                {prefsLoading ? (
+                  <Box sx={{ display: 'flex', justifyContent: 'center', py: 2 }}>
+                    <CircularProgress size={24} />
+                  </Box>
+                ) : (
+                  <FormGroup>
+                    {WIDGETS.map((w) => (
+                      <FormControlLabel
+                        key={w.key}
+                        control={
+                          <Switch
+                            checked={isWidgetVisible(w.key)}
+                            onChange={() => toggleWidget(w.key)}
+                            disabled={mutation.isPending}
+                          />
+                        }
+                        label={w.label}
+                      />
+                    ))}
+                  </FormGroup>
+                )}
+              </CardContent>
+            </Card>
+          </Grid>
+
           {/* Notification Settings */}
           <Grid item xs={12} md={6}>
             <Card>
@@ -146,20 +208,6 @@ function Settings() {
             </Card>
           </Grid>
         </Grid>
-
-        <Box
-          sx={{
-            mt: 3,
-            p: 2,
-            backgroundColor: 'info.light',
-            borderRadius: 1,
-          }}
-        >
-          <Typography variant="body2" color="info.dark">
-            Diese Einstellungen sind derzeit nur zur Demonstration verfügbar. Die Funktionalität
-            wird in zukünftigen Updates implementiert.
-          </Typography>
-        </Box>
       </Container>
     </DashboardLayout>
   );