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/services/setup_service.py

@@ -99,3 +99,19 @@ async def get_password_hash(db: aiosqlite.Connection) -> str | None:
         The bcrypt hash string, or ``None``.
     """
     return await settings_repo.get_setting(db, _KEY_PASSWORD_HASH)
+
+
+async def get_timezone(db: aiosqlite.Connection) -> str:
+    """Return the configured IANA timezone string.
+
+    Falls back to ``"UTC"`` when no timezone has been stored (e.g. before
+    setup completes or for legacy databases).
+
+    Args:
+        db: Active aiosqlite connection.
+
+    Returns:
+        An IANA timezone identifier such as ``"Europe/Berlin"`` or ``"UTC"``.
+    """
+    tz = await settings_repo.get_setting(db, _KEY_TIMEZONE)
+    return tz if tz else "UTC"