Add timezone selector to Admin settings with full-stack support

- Add `timezone` column to User model (VARCHAR 64, default UTC) with
  idempotent startup migration
- Expose and persist timezone via PATCH /api/users/me
- Fix TimerEvent.occurred_at serialization to include UTC offset marker
  (+00:00) so JavaScript correctly parses timestamps as UTC
- Add frontend utility (src/utils/time.js) with timezone-aware
  formatTime, getHHMM, getDateInTZ, tzDateTimeToUTC helpers and a
  curated IANA timezone list
- Add Settings section to Admin page with timezone dropdown; saves to
  both the API and localStorage for the unauthenticated TV view
- Update Activity Log to display and edit times in the user's timezone
- Update TV dashboard clock to respect the saved timezone
- Update README: features, setup steps, usage table, WebSocket events

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

frontend/src/stores/auth.js

@@ -7,6 +7,7 @@ export const useAuthStore = defineStore('auth', () => {
   const user = ref(null)
 
   const isAuthenticated = computed(() => !!accessToken.value)
+  const timezone = computed(() => user.value?.timezone || 'UTC')
 
   function setToken(token) {
     accessToken.value = token
@@ -67,6 +68,7 @@ export const useAuthStore = defineStore('auth', () => {
     accessToken,
     user,
     isAuthenticated,
+    timezone,
     login,
     register,
     logout,