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/prisma/schema.prisma

@@ -430,7 +430,7 @@ model GpsLocationHistory {
   createdAt       DateTime  @default(now())
 
   @@map("gps_location_history")
-  @@index([deviceId, timestamp])
+  @@unique([deviceId, timestamp]) // Prevent duplicate position records
   @@index([timestamp]) // For cleanup job
 }