feat(frontend): add weather icons and redesign calendar header

Backend changes:
- Add LOCATION configuration option to Settings
- Add /api/system/config endpoint to expose public config
- Implement location priority: config > geolocation > default

Frontend changes:
- Install and integrate weather-icons npm package (Erik Flowers)
- Redesign calendar header with date/time on left, weather/location on right
- Display weather icon using CSS classes instead of SVG components
- Fetch location from backend API on component mount
- Use configured location name (from .env) instead of geocoded result

Layout:
- Left: month/year + current time
- Right: city name + weather description + weather icon

backend/app/routers/system.py

@@ -7,3 +7,9 @@ router = APIRouter(prefix='/api/system', tags=['system'])
 @router.get('/status')
 async def get_system_status():
     return SystemService().get_status()
+
+
+@router.get('/config')
+async def get_system_config():
+    """Get public system configuration."""
+    return SystemService().get_config()