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/lib/utils.ts

@@ -5,16 +5,17 @@ export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs));
 }
 
-export function formatDate(date: string | Date): string {
+export function formatDate(date: string | Date, timeZone?: string): string {
   const d = typeof date === 'string' ? new Date(date) : date;
   return d.toLocaleDateString('en-US', {
     year: 'numeric',
     month: 'short',
     day: 'numeric',
+    ...(timeZone && { timeZone }),
   });
 }
 
-export function formatDateTime(date: string | Date): string {
+export function formatDateTime(date: string | Date, timeZone?: string): string {
   const d = typeof date === 'string' ? new Date(date) : date;
   return d.toLocaleString('en-US', {
     year: 'numeric',
@@ -22,13 +23,15 @@ export function formatDateTime(date: string | Date): string {
     day: 'numeric',
     hour: '2-digit',
     minute: '2-digit',
+    ...(timeZone && { timeZone }),
   });
 }
 
-export function formatTime(date: string | Date): string {
+export function formatTime(date: string | Date, timeZone?: string): string {
   const d = typeof date === 'string' ? new Date(date) : date;
   return d.toLocaleTimeString('en-US', {
     hour: '2-digit',
     minute: '2-digit',
+    ...(timeZone && { timeZone }),
   });
 }