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/dashboard/AnnouncementBanner.tsx

@@ -27,7 +27,7 @@ const LEVEL_TITLE: Record<BannerLevel, string> = {
   critical: 'Kritisch',
 };
 
-export default function AnnouncementBanner() {
+export default function AnnouncementBanner({ gridColumn }: { gridColumn?: string }) {
   const [dismissed, setDismissed] = useState<string[]>(() => getDismissed());
 
   const { data: banners = [] } = useQuery({
@@ -48,7 +48,7 @@ export default function AnnouncementBanner() {
   if (visible.length === 0) return null;
 
   return (
-    <Box sx={{ mb: 2, display: 'flex', flexDirection: 'column', gap: 1 }}>
+    <Box sx={{ mb: 0, display: 'flex', flexDirection: 'column', gap: 1, ...(gridColumn ? { gridColumn } : {}) }}>
       {visible.map(banner => (
         <Collapse key={banner.id} in>
           <Alert

frontend/src/components/dashboard/DashboardLayout.tsx

@@ -28,7 +28,7 @@ function DashboardLayoutInner({ children }: DashboardLayoutProps) {
   const chatWidth = chatPanelOpen ? 360 : 60;
 
   return (
-    <Box sx={{ display: 'flex' }}>
+    <Box sx={{ display: 'flex', height: '100vh', overflow: 'hidden' }}>
       <Header onMenuClick={handleDrawerToggle} />
       <Sidebar mobileOpen={mobileOpen} onMobileClose={() => setMobileOpen(false)} />
 
@@ -38,7 +38,7 @@ function DashboardLayoutInner({ children }: DashboardLayoutProps) {
           flexGrow: 1,
           p: 3,
           width: { sm: `calc(100% - ${sidebarWidth}px - ${chatWidth}px)` },
-          minHeight: '100vh',
+          overflowY: 'auto',
           backgroundColor: 'background.default',
           transition: 'width 225ms cubic-bezier(0.4, 0, 0.6, 1)',
           minWidth: 0,

frontend/src/components/dashboard/LinksWidget.tsx

@@ -0,0 +1,63 @@
+import {
+  Card,
+  CardContent,
+  Typography,
+  Link,
+  Stack,
+  Box,
+  Divider,
+} from '@mui/material';
+import { Link as LinkIcon, OpenInNew } from '@mui/icons-material';
+import { useQuery } from '@tanstack/react-query';
+import { configApi } from '../../services/config';
+
+function LinksWidget() {
+  const { data: externalLinks } = useQuery({
+    queryKey: ['external-links'],
+    queryFn: () => configApi.getExternalLinks(),
+    staleTime: 10 * 60 * 1000,
+  });
+
+  const collections = externalLinks?.linkCollections ?? [];
+  const nonEmpty = collections.filter((c) => c.links.length > 0);
+
+  if (nonEmpty.length === 0) return null;
+
+  return (
+    <>
+      {nonEmpty.map((collection) => (
+        <Card key={collection.id}>
+          <CardContent>
+            <Box sx={{ display: 'flex', alignItems: 'center', mb: 1 }}>
+              <LinkIcon color="primary" sx={{ mr: 1 }} />
+              <Typography variant="h6">{collection.name}</Typography>
+            </Box>
+            <Divider sx={{ mb: 1.5 }} />
+            <Stack spacing={0.5}>
+              {collection.links.map((link, i) => (
+                <Link
+                  key={i}
+                  href={link.url}
+                  target="_blank"
+                  rel="noopener noreferrer"
+                  underline="hover"
+                  sx={{
+                    display: 'flex',
+                    alignItems: 'center',
+                    gap: 0.5,
+                    py: 0.5,
+                  }}
+                >
+                  {link.name}
+                  <OpenInNew sx={{ fontSize: 14, opacity: 0.6 }} />
+                </Link>
+              ))}
+            </Stack>
+          </CardContent>
+        </Card>
+      ))}
+    </>
+  );
+}
+
+export default LinksWidget;

frontend/src/components/dashboard/VehicleBookingQuickAddWidget.tsx

@@ -152,6 +152,7 @@ const VehicleBookingQuickAddWidget: React.FC = () => {
               onChange={(e) => setBeginn(e.target.value)}
               required
               InputLabelProps={{ shrink: true }}
+              sx={{ '& input': { color: 'text.primary' } }}
             />
 
             <TextField
@@ -163,6 +164,7 @@ const VehicleBookingQuickAddWidget: React.FC = () => {
               onChange={(e) => setEnde(e.target.value)}
               required
               InputLabelProps={{ shrink: true }}
+              sx={{ '& input': { color: 'text.primary' } }}
             />
 
             <TextField

frontend/src/components/dashboard/index.ts

@@ -13,3 +13,4 @@ export { default as AdminStatusWidget } from './AdminStatusWidget';
 export { default as VehicleBookingQuickAddWidget } from './VehicleBookingQuickAddWidget';
 export { default as EventQuickAddWidget } from './EventQuickAddWidget';
 export { default as AnnouncementBanner } from './AnnouncementBanner';
+export { default as LinksWidget } from './LinksWidget';