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/backend/main.go

@@ -11,7 +11,7 @@ import (
 func main() {
 	cfg := config.Load()
 	db := database.Connect(cfg)
-	r := router.New(db, cfg.JWTSecret)
+	r := router.New(db, cfg)
 	log.Printf("Party Mix backend starting on :%s", cfg.Port)
 	if err := r.Run(":" + cfg.Port); err != nil {
 		log.Fatalf("server failed: %v", err)