improved library scan speeds, added library manager component

-libraries tab now opens a library manager
-choose library directory, manually soft scan and rescan
-batched db queues
-mp3 header-based duration extraction

backend/metadata/duration.go

@@ -0,0 +1,36 @@
+package metadata
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// getTrackDuration returns the duration of an audio file in
+// milliseconds.  For MP3 files it uses a fast header-only parser
+// (Xing/VBRI/CBR); for other formats it falls back to a full
+// decode via beep which is already O(1) for FLAC, OGG, and WAV.
+//
+// The file position is undefined after this call.
+func getTrackDuration(f *os.File) (int64, error) {
+	ext := filepath.Ext(f.Name())
+
+	if ext == ".mp3" {
+		return getMP3Duration(f)
+	}
+
+	// FLAC, OGG, and WAV: beep's Decode() + Len() is already
+	// cheap (reads headers/metadata only, no full audio decode).
+	streamer, format, err := DecodeFile(f)
+	if err != nil {
+		return 0, fmt.Errorf("error decoding file: %w", err)
+	}
+
+	lengthMillis := int64(
+		float64(streamer.Len()*1000) /
+			float64(format.SampleRate),
+	)
+	_ = streamer.Close()
+
+	return lengthMillis, nil
+}