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

frontend/package.json

@@ -10,8 +10,8 @@
     "test": "vitest run"
   },
   "dependencies": {
-    "3d-force-graph": "^1.79.0",
     "@vueuse/core": "^14.2.1",
+    "3d-force-graph": "^1.79.0",
     "axios": "^1.13.6",
     "echarts": "^6.0.0",
     "element-plus": "^2.13.6",
@@ -20,7 +20,8 @@
     "pinia": "^3.0.4",
     "three": "^0.180.0",
     "vue": "^3.5.30",
-    "vue-router": "^4.6.4"
+    "vue-router": "^4.6.4",
+    "weather-icons": "^1.3.2"
   },
   "devDependencies": {
     "@types/node": "^25.5.0",