fix: dashboard layout, widget caching, and backend stability

Layout:
- Remove Container maxWidth cap so widgets scale fluidly on wide screens
- Fix ActivityFeed Card missing height:100% and overflow:hidden that caused
  the timeline connector pseudo-element to bleed outside the card boundary

Performance (frontend):
- Migrate VehicleDashboardCard, EquipmentDashboardCard, AtemschutzDashboardCard,
  UpcomingEventsWidget, and PersonalWarningsBanner from useEffect+useState to
  TanStack Query — cached for 5 min, so navigating back to the dashboard no
  longer re-fires all 9 API requests
- Add gcTime:10min and refetchOnWindowFocus:false to QueryClient defaults to
  prevent spurious refetches on tab-switch

Backend stability:
- Raise default RATE_LIMIT_MAX from 100 to 300 req/15min — the previous limit
  was easily exceeded by a single active user during normal dashboard navigation
- Increase DB connectionTimeoutMillis from 2s to 5s to handle burst-load
  scenarios where multiple requests compete for pool slots simultaneously

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/config/database.ts

@@ -16,7 +16,7 @@ const poolConfig: PoolConfig = {
   password: environment.database.password,
   max: 20, // Maximum number of clients in the pool
   idleTimeoutMillis: 30000, // Close idle clients after 30 seconds
-  connectionTimeoutMillis: 2000, // Return an error if connection takes longer than 2 seconds
+  connectionTimeoutMillis: 5000, // Return an error if connection takes longer than 5 seconds
 };
 
 const pool = new Pool(poolConfig);