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/nginx/nginx.conf

@@ -0,0 +1,68 @@
+events {
+    worker_connections 1024;
+}
+
+http {
+    # SSE and MP3 streaming — no buffering, long timeouts
+    upstream backend {
+        server backend:8080;
+    }
+
+    upstream web {
+        server web:3000;
+    }
+
+    server {
+        listen 80;
+
+        client_max_body_size 16m;
+
+        # SSE overlay stream
+        location ~ ^/api/overlay/[^/]+/stream {
+            proxy_pass http://backend;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_buffering off;
+            proxy_cache off;
+            proxy_read_timeout 120s;
+            proxy_set_header Connection '';
+            chunked_transfer_encoding on;
+        }
+
+        # MP3 proxy — large bodies, streaming
+        location /api/proxy/mp3 {
+            proxy_pass http://backend;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header Range $http_range;
+            proxy_set_header If-Range $http_if_range;
+            proxy_buffering off;
+            proxy_cache off;
+            proxy_read_timeout 300s;
+        }
+
+        # All other API calls
+        location /api/ {
+            proxy_pass http://backend;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_read_timeout 30s;
+        }
+
+        # Next.js app (with WebSocket for hot reload in dev)
+        location / {
+            proxy_pass http://web;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header Upgrade $http_upgrade;
+            proxy_set_header Connection 'upgrade';
+            proxy_read_timeout 60s;
+        }
+    }
+}