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

src/vite-env.d.ts

@@ -16,6 +16,7 @@ interface ViteTypeOptions {
 
 interface ImportMetaEnv {
     readonly VITE_SERVER_URL_DEFAULT: string;
+    readonly VITE_SUBPATH?: string;
     // more env variables...
 }