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/services/system_service.py

@@ -1,5 +1,6 @@
 from datetime import datetime, UTC
 from time import monotonic
+import os
 import platform
 import socket
 import subprocess
@@ -15,6 +16,11 @@ class SystemService:
     _last_net_bytes_recv: int | None = None
     _last_net_sample_at: float | None = None
 
+    def __init__(self):
+        # Import settings here to avoid circular imports
+        from app.config import settings
+        self._settings = settings
+
     def _get_network_rates(self) -> tuple[float, float]:
         counters = psutil.net_io_counters()
         now = monotonic()
@@ -127,3 +133,9 @@ class SystemService:
             **gpu_status,
             'timestamp': datetime.now(UTC).isoformat(),
         }
+
+    def get_config(self) -> dict:
+        """Get public system configuration."""
+        return {
+            'location': self._settings.LOCATION,
+        }