feat: add PDF reports, timezone management, GPS QR codes, and fix GPS tracking gaps

Issue #1: QR button on GPS Devices tab for re-enrollment
Issue #2: App-wide timezone setting with TimezoneContext, useFormattedDate hook,
  and admin timezone selector. All date displays now respect the configured timezone.
Issue #3: PDF export for Accountability Roster using @react-pdf/renderer with
  professional styling matching VIPSchedulePDF. Added Signal send button.
Issue #4: Fixed GPS "teleporting" gaps - syncPositions now fetches position history
  per device instead of only latest position. Changed cron to every 30s, added
  unique constraint on deviceId+timestamp for deduplication, lowered min interval to 10s.

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

frontend/src/hooks/useGps.ts

@@ -8,6 +8,7 @@ import type {
   GpsSettings,
   EnrollmentResponse,
   MyGpsStatus,
+  DeviceQrInfo,
 } from '@/types/gps';
 import toast from 'react-hot-toast';
 
@@ -78,6 +79,20 @@ export function useGpsDevices() {
   });
 }
 
+/**
+ * Get QR code info for an enrolled device (on demand)
+ */
+export function useDeviceQr(driverId: string | null) {
+  return useQuery<DeviceQrInfo>({
+    queryKey: ['gps', 'devices', driverId, 'qr'],
+    queryFn: async () => {
+      const { data } = await api.get(`/gps/devices/${driverId}/qr`);
+      return data;
+    },
+    enabled: !!driverId,
+  });
+}
+
 /**
  * Get all active driver locations (for map)
  */
@@ -88,7 +103,7 @@ export function useDriverLocations() {
       const { data } = await api.get('/gps/locations');
       return data;
     },
-    refetchInterval: 30000, // Refresh every 30 seconds
+    refetchInterval: 15000, // Refresh every 15 seconds
   });
 }
 
@@ -103,7 +118,7 @@ export function useDriverLocation(driverId: string) {
       return data;
     },
     enabled: !!driverId,
-    refetchInterval: 30000,
+    refetchInterval: 15000,
   });
 }