BASE_URL support

[cyrus28214]

Enable subpath deployment via BASE_URL for easier reverse proxy/subdirectory hosting. Close #198.

Changes:

1. vite.config.ts: set base from BASE_URL
2. Router: BrowserRouter basename={import.meta.env.BASE_URL}
3. i18n: loadPath prefixed by BASE_URL
4. Assets: background via imports; fonts imported in src/index.tsx

Usage:

export BASE_URL='/omni-tools/'
npm run dev
# or
npm run build
npm run serve

Help Needed:

1. Can anyone help me check if PWA and Docker works well with BASE_URL?
2. I am not sure if I've caught all the absolute paths.