feat: configurable base path via BASE_PATH env var

[4mgad]

Summary

• Add support for serving the application under a configurable sub-path prefix via the BASE_PATH environment variable (defaults to /)
• All routes (API, WebSocket, static assets), React Router basename, service worker, and PWA manifest resolve correctly under the configured prefix
• A root / → BASE_PATH/ redirect is added automatically when a non-root base path is set

Details

• Backend: Express routes, WebSocket pathname checks, static file serving, and SPA fallback are all scoped to BASE_PATH
• Frontend: authenticatedFetch auto-prefixes API calls; WebSocket URLs, logo assets, EventSource, and Router basename all derive from import.meta.env.BASE_URL
• Service worker: derives its base path at runtime from self.location
• PWA manifest: uses relative paths so icons and scope resolve correctly for any prefix
• Vite config: reads BASE_PATH from env, sets base and proxy rules accordingly

Test plan

• Run npm run dev without BASE_PATH set — app should work at http://localhost:5173/ (original behavior)
• Set BASE_PATH=/cloudcli in .env, restart dev server — app should work at http://localhost:5173/cloudcli/
• Verify API calls, WebSocket connections, shell terminal, file tree, and git panel all function under the prefix
• Run npm start and verify production mode serves correctly under the configured prefix
• Verify PWA install prompt and push notification icons load correctly

Summary by CodeRabbit

• New Features
  • App can be deployed under a configurable base path; routing, asset URLs, service worker, WebSocket and API endpoints, manifest, and startup links respect it.
• Bug Fixes
  • Improved offline behavior and scoped caching for reliable updates and navigation when served from a non-root path; manifest and icon URLs now resolve correctly.
• Chores
  • Improved plugin installation handling to better support prebuilt native artifacts.