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/components/EventForm.tsx

@@ -4,7 +4,7 @@ import { useQuery } from '@tanstack/react-query';
 import { X, AlertTriangle, Users, Car, Link2 } from 'lucide-react';
 import { api } from '@/lib/api';
 import { ScheduleEvent, VIP, Driver, Vehicle } from '@/types';
-import { formatDateTime } from '@/lib/utils';
+import { useFormattedDate } from '@/hooks/useFormattedDate';
 
 interface EventFormProps {
   event?: ScheduleEvent | null;
@@ -39,6 +39,8 @@ interface ScheduleConflict {
 }
 
 export function EventForm({ event, onSubmit, onCancel, isSubmitting, extraActions }: EventFormProps) {
+  const { formatDateTime } = useFormattedDate();
+
   // Helper to convert ISO datetime to datetime-local format
   const toDatetimeLocal = (isoString: string | null | undefined) => {
     if (!isoString) return '';