feat: nginx reverse proxy, Spotify import, overlay system, UI overhaul

- Add nginx as single entry point: /api/* → backend, /* → web
- NEXT_PUBLIC_API_URL="" so all API calls are relative (go through nginx)
- Add Spotify playlist import (Client Credentials OAuth, up to 500 tracks)
- Add Yandex/Spotify tabbed import UI on /playlists
- Add stream overlay system (SSE + polling fallback, 9 styles)
- Reorganize pages into (main) route group
- Add QueuePanel, VersionsPanel, Toaster components
- Add overlay settings tab in /settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/next-env.d.ts

@@ -0,0 +1,6 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+/// <reference path="./.next/types/routes.d.ts" />
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.