Add dynamic subpath support (#1011)

* Add dynamic subpath support for WebUI

- Add SubpathConfig utility for server-side subpath detection
- Configure React Router basename from server config
- Update API base URL handling for subpath compatibility
- Fix asset loading with relative paths
- Add VITE_SUBPATH environment variable for development
- Update locales and manifest paths for subpath support

Closes #174

* Move VITE_SUBPATH interface to vite-env.d.ts

* Ensure production bundle always uses relative base

public/site.webmanifest

@@ -4,18 +4,18 @@
   "start_url": ".",
   "icons": [
     {
-      "src": "/web-app-manifest-192x192.png",
+      "src": "./web-app-manifest-192x192.png",
       "sizes": "192x192",
       "type": "image/png"
     },
     {
-      "src": "/web-app-manifest-192x192.png",
+      "src": "./web-app-manifest-192x192.png",
       "sizes": "192x192",
       "type": "image/png",
       "purpose": "maskable"
     },
     {
-      "src": "/web-app-manifest-512x512.png",
+      "src": "./web-app-manifest-512x512.png",
       "sizes": "512x512",
       "type": "image/png",
       "purpose": "maskable"