Stage 11: polish, cross-cutting concerns & hardening

- 11.1 MainLayout health indicator: warning MessageBar when fail2ban offline
- 11.2 formatDate utility + TimezoneProvider + GET /api/setup/timezone
- 11.3 Responsive sidebar: auto-collapse <640px, media query listener
- 11.4 PageFeedback (PageLoading/PageError/PageEmpty), BanTable updated
- 11.5 prefers-reduced-motion: disable sidebar transition
- 11.6 WorldMap ARIA: role/tabIndex/aria-label/onKeyDown for countries
- 11.7 Health transition logging (fail2ban_came_online/went_offline)
- 11.8 Global handlers: Fail2BanConnectionError/ProtocolError -> 502
- 11.9 379 tests pass, 82% coverage, ruff+mypy+tsc+eslint clean
- Timezone endpoint: setup_service.get_timezone, 5 new tests

backend/app/models/setup.py

@@ -45,6 +45,14 @@ class SetupResponse(BaseModel):
     )
 
 
+class SetupTimezoneResponse(BaseModel):
+    """Response for ``GET /api/setup/timezone``."""
+
+    model_config = ConfigDict(strict=True)
+
+    timezone: str = Field(..., description="Configured IANA timezone identifier.")
+
+
 class SetupStatusResponse(BaseModel):
     """Response indicating whether setup has been completed."""