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/Dockerfile

@@ -9,6 +9,7 @@ COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 ARG NEXT_PUBLIC_API_URL=http://localhost:8080
 ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
+ENV NEXT_TELEMETRY_DISABLED=1
 RUN npm run build
 
 FROM node:22-alpine AS runner