Add a mini-player to build mode

Every song in the Build tab gets a play button. A bar along the bottom
plays the folder in order with seeking, previous/next, volume (kept per
browser) and space to pause, and says so when the browser can't play a
format. Songs stream from the master library through a new endpoint with
range requests; it only serves regular music files inside the library.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -101,6 +101,7 @@ func (s *Server) Handler() http.Handler {
 				r.Post("/renames", s.handleRenames)
 				r.Post("/link-identical", s.handleLinkIdentical)
 				r.Get("/tree", s.handleTree)
+				r.Get("/listen", s.handleListen)
 				r.Get("/empty-folders", s.handleEmptyFolders)
 				r.Post("/empty-folders/scan", s.handleEmptyFolderScan)
 				r.Post("/empty-folders/remove", s.handleRemoveEmptyFolders)
@@ -786,3 +787,33 @@ func (s *Server) handleRemoveEmptyFolders(w http.ResponseWriter, r *http.Request
 	s.log.Info("removing folders without music", "pair", id, "folders", len(body.Folders))
 	writeJSON(w, 202, j)
 }
+
+// ---- listening (build mode player) ----
+
+var audioTypes = map[string]string{
+	".flac": "audio/flac", ".mp3": "audio/mpeg", ".m4a": "audio/mp4", ".aac": "audio/aac", ".alac": "audio/mp4",
+	".ogg": "audio/ogg", ".oga": "audio/ogg", ".opus": "audio/ogg", ".wav": "audio/wav", ".aif": "audio/aiff",
+	".aiff": "audio/aiff", ".mka": "audio/x-matroska", ".wma": "audio/x-ms-wma",
+}
+
+// handleListen streams a song with range support, so the browser can seek.
+func (s *Server) handleListen(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	q := r.URL.Query()
+	f, fi, err := s.eng.OpenSong(r.Context(), id, q.Get("side"), q.Get("path"))
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	defer f.Close()
+	ct := audioTypes[strings.ToLower(filepath.Ext(fi.Name()))]
+	if ct == "" {
+		ct = "application/octet-stream"
+	}
+	w.Header().Set("Content-Type", ct)
+	w.Header().Set("Cache-Control", "private, max-age=300")
+	// A long song on a slow connection may take longer than the server's
+	// write timeout.
+	_ = http.NewResponseController(w).SetWriteDeadline(time.Time{})
+	http.ServeContent(w, r, fi.Name(), fi.ModTime(), f)
+}