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

vite.config.ts

@@ -17,7 +17,8 @@ import { nodePolyfills } from 'vite-plugin-node-polyfills';
 import 'dotenv/config';
 
 // eslint-disable-next-line import/no-default-export
-export default defineConfig(() => ({
+export default defineConfig(({ command }) => ({
+    base: command === 'serve' ? process.env.VITE_SUBPATH || './' : './',
     build: {
         outDir: 'build',
     },