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>

frontend/src/main.tsx

@@ -10,7 +10,9 @@ const queryClient = new QueryClient({
   defaultOptions: {
     queries: {
       staleTime: 5 * 60 * 1000, // 5 minutes
+      gcTime: 10 * 60 * 1000,   // keep cache 10 minutes
       retry: 1,
+      refetchOnWindowFocus: false, // prevent refetch on every tab switch
     },
   },
 });