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/services/nextcloud.service.ts

@@ -202,7 +202,7 @@ async function getMessages(token: string, loginName: string, appPassword: string
 
   try {
     const response = await axios.get(
-      `${baseUrl}/ocs/v2.php/apps/spreed/api/v4/chat/${encodeURIComponent(token)}`,
+      `${baseUrl}/ocs/v2.php/apps/spreed/api/v1/chat/${encodeURIComponent(token)}`,
       {
         params: { lookIntoFuture: 0, limit: 50, setReadMarker: 0 },
         headers: {
@@ -251,7 +251,7 @@ async function sendMessage(token: string, message: string, loginName: string, ap
 
   try {
     await axios.post(
-      `${baseUrl}/ocs/v2.php/apps/spreed/api/v4/chat/${encodeURIComponent(token)}`,
+      `${baseUrl}/ocs/v2.php/apps/spreed/api/v1/chat/${encodeURIComponent(token)}`,
       { message },
       {
         headers: {
@@ -288,7 +288,7 @@ async function markAsRead(token: string, loginName: string, appPassword: string)
 
   try {
     await axios.delete(
-      `${baseUrl}/ocs/v2.php/apps/spreed/api/v4/chat/${encodeURIComponent(token)}/read`,
+      `${baseUrl}/ocs/v2.php/apps/spreed/api/v1/chat/${encodeURIComponent(token)}/read`,
       {
         headers: {
           'Authorization': `Basic ${Buffer.from(loginName + ':' + appPassword).toString('base64')}`,