fix: extract cover art from M4A/ALAC files for conversion

Add extractCoverFromM4A() that reads the covr atom from the MP4
box tree (moov/udta/meta/ilst/covr/data). Wire it into
ExtractCoverToFile so ALAC-to-FLAC conversion preserves cover art.

go_backend/exports.go

@@ -1694,6 +1694,8 @@ func ExtractCoverToFile(audioPath string, outputPath string) error {
 
 	if strings.HasSuffix(lower, ".flac") {
 		coverData, err = ExtractCoverArt(audioPath)
+	} else if strings.HasSuffix(lower, ".m4a") || strings.HasSuffix(lower, ".aac") {
+		coverData, err = extractCoverFromM4A(audioPath)
 	} else if strings.HasSuffix(lower, ".mp3") {
 		coverData, _, err = extractMP3CoverArt(audioPath)
 	} else if strings.HasSuffix(lower, ".opus") || strings.HasSuffix(lower, ".ogg") {

go_backend/metadata.go

@@ -663,6 +663,79 @@ func extractLyricsFromM4A(filePath string) (string, error) {
 	return string(buf), nil
 }
 
+func extractCoverFromM4A(filePath string) ([]byte, error) {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	fi, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	fileSize := fi.Size()
+
+	moov, found, err := findAtomInRange(f, 0, fileSize, "moov", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("moov not found")
+	}
+
+	bodyStart := moov.offset + moov.headerSize
+	bodySize := moov.size - moov.headerSize
+
+	udta, found, err := findAtomInRange(f, bodyStart, bodySize, "udta", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("udta not found")
+	}
+
+	bodyStart = udta.offset + udta.headerSize
+	bodySize = udta.size - udta.headerSize
+
+	meta, found, err := findAtomInRange(f, bodyStart, bodySize, "meta", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("meta not found")
+	}
+
+	bodyStart = meta.offset + meta.headerSize + 4
+	bodySize = meta.size - meta.headerSize - 4
+
+	ilst, found, err := findAtomInRange(f, bodyStart, bodySize, "ilst", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("ilst not found")
+	}
+
+	bodyStart = ilst.offset + ilst.headerSize
+	bodySize = ilst.size - ilst.headerSize
+
+	covr, found, err := findAtomInRange(f, bodyStart, bodySize, "covr", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("cover atom not found")
+	}
+
+	dataStart := covr.offset + covr.headerSize
+	dataSize := covr.size - covr.headerSize
+
+	dataAtom, found, err := findAtomInRange(f, dataStart, dataSize, "data", fileSize)
+	if err != nil || !found {
+		return nil, fmt.Errorf("data atom not found in cover")
+	}
+
+	// data atom: header + 4 bytes type indicator + 4 bytes locale
+	imgStart := dataAtom.offset + dataAtom.headerSize + 8
+	imgLen := dataAtom.size - dataAtom.headerSize - 8
+	if imgLen <= 0 {
+		return nil, fmt.Errorf("empty cover data")
+	}
+
+	buf := make([]byte, imgLen)
+	if _, err := f.ReadAt(buf, imgStart); err != nil {
+		return nil, err
+	}
+
+	return buf, nil
+}
+
 func extractLyricsFromSidecarLRC(filePath string) (string, error) {
 	ext := filepath.Ext(filePath)
 	base := strings.TrimSuffix(filePath, ext)