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/bookstack.service.ts

@@ -98,7 +98,7 @@ async function getRecentPages(): Promise<BookStackPage[]> {
     const pages: BookStackPage[] = response.data?.data ?? [];
     return pages.map((p) => ({
       ...p,
-      url: p.url && p.url.startsWith('http') ? p.url : `${bookstack.url}/books/${p.book_slug}/page/${p.slug}`,
+      url: p.url && p.url.startsWith('http') ? p.url : `${bookstack.url}/books/${p.book_slug || p.book_id}/page/${p.slug}`,
     }));
   } catch (error) {
     if (axios.isAxiosError(error)) {
@@ -134,7 +134,7 @@ async function searchPages(query: string): Promise<BookStackSearchResult[]> {
         slug: item.slug,
         book_id: item.book_id ?? 0,
         book_slug: item.book_slug ?? '',
-        url: item.url || `${bookstack.url}/books/${item.book_slug}/page/${item.slug}`,
+        url: item.url || `${bookstack.url}/books/${item.book_slug || item.book_id}/page/${item.slug}`,
         preview_html: item.preview_html ?? { content: '' },
         tags: item.tags ?? [],
       }));
@@ -189,7 +189,7 @@ async function getPageById(id: number): Promise<BookStackPageDetail> {
       html: page.html ?? '',
       created_at: page.created_at,
       updated_at: page.updated_at,
-      url: page.url && page.url.startsWith('http') ? page.url : `${bookstack.url}/books/${page.book_slug}/page/${page.slug}`,
+      url: page.url && page.url.startsWith('http') ? page.url : `${bookstack.url}/books/${page.book_slug || page.book_id}/page/${page.slug}`,
       book: page.book,
       createdBy: page.created_by,
       updatedBy: page.updated_by,