fix(17-02): fix cover art replace and remove

Three issues fixed:

1. asBytes() helper for []interface{} → []byte conversion — same
   float64 deserialization issue as numeric fields. Cover art data
   from the frontend arrives as []interface{} of float64, not []byte.

2. DB sync for cover art — was a placeholder no-op. Now saves image
   to covers cache dir (content-hash dedup + thumbnail generation),
   upserts cover_art row, and updates release_groups.cover_art_id.
   Clear sets cover_art_id to NULL on linked release groups.

3. Frontend ReadFile returns base64 string (Go []byte JSON encoding),
   not number[]. Decode with atob() before creating Uint8Array for
   preview blob URL.

backend/tagwriter/flac.go

@@ -173,7 +173,7 @@ func applyFlacCoverArt(f *flac.File, changes TagChanges) error {
 	f.Meta = newMeta
 
 	// If value is nil or empty, we've cleared the art — done.
-	data, isBytes := v.([]byte)
+	data, isBytes := asBytes(v)
 	if !isBytes || len(data) == 0 {
 		return nil
 	}