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/serviceMonitor.service.ts

@@ -18,6 +18,7 @@ export interface PingResult {
   status: 'up' | 'down';
   latencyMs: number;
   error?: string;
+  checked_at: string;
 }
 
 export interface StatusSummary {
@@ -92,6 +93,7 @@ class ServiceMonitorService {
         url,
         status: 'up',
         latencyMs: Date.now() - start,
+        checked_at: new Date().toISOString(),
       };
     } catch (error) {
       // Treat any HTTP response (even 4xx) as "service is reachable" only for status.php-type endpoints
@@ -105,6 +107,7 @@ class ServiceMonitorService {
             url,
             status: 'up',
             latencyMs: Date.now() - start,
+            checked_at: new Date().toISOString(),
           };
         }
       }
@@ -113,6 +116,7 @@ class ServiceMonitorService {
         url,
         status: 'down',
         latencyMs: Date.now() - start,
+        checked_at: new Date().toISOString(),
         error: axios.isAxiosError(error)
           ? `${error.code ?? 'ERROR'}: ${error.message}`
           : String(error),
@@ -140,9 +144,37 @@ class ServiceMonitorService {
       })
     );
 
+    // Store ping results in history (fire-and-forget)
+    this.storePingResults(results).catch(() => {});
+
     return results;
   }
 
+  async getPingHistory(serviceId: string): Promise<Array<{ id: number; service_id: string; status: string; response_time_ms: number | null; checked_at: string }>> {
+    const result = await pool.query(
+      'SELECT * FROM service_ping_history WHERE service_id = $1 ORDER BY checked_at DESC LIMIT 20',
+      [serviceId]
+    );
+    return result.rows;
+  }
+
+  private async storePingResults(results: PingResult[]): Promise<void> {
+    for (const r of results) {
+      const serviceId = r.name || r.url;
+      await pool.query(
+        'INSERT INTO service_ping_history (service_id, status, response_time_ms, checked_at) VALUES ($1, $2, $3, $4)',
+        [serviceId, r.status, r.latencyMs, r.checked_at]
+      );
+      // Keep only last 20 per service
+      await pool.query(
+        `DELETE FROM service_ping_history WHERE service_id = $1 AND id NOT IN (
+          SELECT id FROM service_ping_history WHERE service_id = $1 ORDER BY checked_at DESC LIMIT 20
+        )`,
+        [serviceId]
+      );
+    }
+  }
+
   async getStatusSummary(): Promise<StatusSummary> {
     const results = await this.pingAll();
     return {