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-lock.json

@@ -18,7 +18,8 @@
         "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",
@@ -3853,6 +3854,12 @@
         "node": ">=18"
       }
     },
+    "node_modules/weather-icons": {
+      "version": "1.3.2",
+      "resolved": "https://registry.npmjs.org/weather-icons/-/weather-icons-1.3.2.tgz",
+      "integrity": "sha512-EW/i0gFzlPpyBK29iXtruDw1v9txI4Gjp7q6ikbFTsL6O9gt3yH64/fuMf7wxpj92v5I08JhajBra/wMn715Eg==",
+      "license": "MIT"
+    },
     "node_modules/webidl-conversions": {
       "version": "8.0.1",
       "resolved": "https://registry.npmmirror.com/webidl-conversions/-/webidl-conversions-8.0.1.tgz",