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/config.py

@@ -61,6 +61,9 @@ class Settings(BaseSettings):
     DAILY_PLAN_TIME: str = "00:00"
     FORUM_SCAN_INTERVAL_MINUTES: int = 30
 
+    # === 位置配置 ===
+    LOCATION: str = "Location"
+
     # === CORS ===
     CORS_ORIGINS: list[str] = ["http://localhost:5173", "http://localhost:3000"]