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>

backend/app/schemas/activity.py

@@ -1,5 +1,5 @@
-from datetime import date, datetime
-from pydantic import BaseModel
+from datetime import date, datetime, timezone
+from pydantic import BaseModel, field_serializer
 
 
 class ActivityLogCreate(BaseModel):
@@ -45,3 +45,9 @@ class TimelineEventOut(BaseModel):
     subject_name: str | None = None
     subject_icon: str | None = None
     subject_color: str | None = None
+
+    @field_serializer("occurred_at")
+    def serialize_occurred_at(self, dt: datetime) -> str:
+        if dt.tzinfo is None:
+            dt = dt.replace(tzinfo=timezone.utc)
+        return dt.isoformat()

backend/app/schemas/user.py

@@ -7,6 +7,7 @@ class UserOut(BaseModel):
     full_name: str
     is_active: bool
     is_admin: bool
+    timezone: str = "UTC"
 
     model_config = {"from_attributes": True}
 
@@ -14,3 +15,4 @@ class UserOut(BaseModel):
 class UserUpdate(BaseModel):
     full_name: str | None = None
     email: EmailStr | None = None
+    timezone: str | None = None