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>

backend/src/gps/gps.controller.ts

@@ -78,6 +78,15 @@ export class GpsController {
     return this.gpsService.getEnrolledDevices();
   }
 
+  /**
+   * Get QR code info for an enrolled device
+   */
+  @Get('devices/:driverId/qr')
+  @Roles(Role.ADMINISTRATOR)
+  async getDeviceQr(@Param('driverId') driverId: string) {
+    return this.gpsService.getDeviceQrInfo(driverId);
+  }
+
   /**
    * Enroll a driver for GPS tracking
    */