fix: resolve album-only autofill and placeholder re-enrich regressions

- Dart: _metadataMatchIsConfident now handles album-only case (title empty)
  by adding albumMatches fallback branch
- Go: selectBestReEnrichTrack treats placeholder values (Unknown Title,
  Unknown Artist) as empty via isPlaceholderReEnrichValue, so album-based
  fallback filtering works correctly
- Add test for placeholder album fallback in selectBestReEnrichTrack

go_backend/exports.go

@@ -594,6 +594,14 @@ func selectBestReEnrichTrack(req reEnrichRequest, tracks []ExtTrackMetadata) *Ex
 	downloadReq := reEnrichDownloadRequest(req)
 	currentISRC := strings.TrimSpace(req.ISRC)
 	currentAlbum := strings.TrimSpace(req.AlbumName)
+	effectiveTrackName := req.TrackName
+	if isPlaceholderReEnrichValue(effectiveTrackName) {
+		effectiveTrackName = ""
+	}
+	effectiveArtistName := req.ArtistName
+	if isPlaceholderReEnrichValue(effectiveArtistName) {
+		effectiveArtistName = ""
+	}
 	var best *ExtTrackMetadata
 	bestScore := -1 << 30
 
@@ -601,8 +609,8 @@ func selectBestReEnrichTrack(req reEnrichRequest, tracks []ExtTrackMetadata) *Ex
 		track := &tracks[i]
 		score := 0
 		exactISRCMatch := currentISRC != "" && strings.EqualFold(currentISRC, strings.TrimSpace(track.ISRC))
-		titleMatches := req.TrackName != "" && track.Name != "" && titlesMatch(req.TrackName, track.Name)
-		artistMatches := req.ArtistName != "" && track.Artists != "" && artistsMatch(req.ArtistName, track.Artists)
+		titleMatches := effectiveTrackName != "" && track.Name != "" && titlesMatch(effectiveTrackName, track.Name)
+		artistMatches := effectiveArtistName != "" && track.Artists != "" && artistsMatch(effectiveArtistName, track.Artists)
 		albumMatches := currentAlbum != "" && track.AlbumName != "" && titlesMatch(currentAlbum, track.AlbumName)
 
 		resolved := resolvedTrackInfo{
@@ -614,16 +622,16 @@ func selectBestReEnrichTrack(req reEnrichRequest, tracks []ExtTrackMetadata) *Ex
 		verified := trackMatchesRequest(downloadReq, resolved, "ReEnrich")
 
 		if !exactISRCMatch {
-			if req.TrackName != "" && !titleMatches {
+			if effectiveTrackName != "" && !titleMatches {
 				continue
 			}
-			if req.ArtistName != "" && !artistMatches {
+			if effectiveArtistName != "" && !artistMatches {
 				continue
 			}
-			if req.TrackName == "" && req.ArtistName == "" && currentAlbum != "" && !albumMatches {
+			if effectiveTrackName == "" && effectiveArtistName == "" && currentAlbum != "" && !albumMatches {
 				continue
 			}
-			if req.TrackName == "" && req.ArtistName == "" && currentAlbum == "" && !verified {
+			if effectiveTrackName == "" && effectiveArtistName == "" && currentAlbum == "" && !verified {
 				continue
 			}
 		}

go_backend/exports_test.go

@@ -463,6 +463,34 @@ func TestSelectBestReEnrichTrackAllowsExactISRCDespiteMetadataMismatch(t *testin
 	}
 }
 
+func TestSelectBestReEnrichTrackPlaceholderFallsBackToAlbum(t *testing.T) {
+	req := reEnrichRequest{
+		TrackName:  "Unknown Title",
+		ArtistName: "Unknown Artist",
+		AlbumName:  "Harry Styles",
+		DurationMs: 180000,
+	}
+
+	tracks := []ExtTrackMetadata{
+		{
+			ID:         "album-match",
+			Name:       "Sign of the Times",
+			Artists:    "Harry Styles",
+			AlbumName:  "Harry Styles",
+			DurationMS: 180000,
+			ProviderID: "deezer",
+		},
+	}
+
+	best := selectBestReEnrichTrack(req, tracks)
+	if best == nil {
+		t.Fatal("expected album-matching candidate to be selected when title/artist are placeholders")
+	}
+	if best.ID != "album-match" {
+		t.Fatalf("selected track = %q, want album-match", best.ID)
+	}
+}
+
 func TestBuildReEnrichFFmpegMetadataOmitsEmptyFields(t *testing.T) {
 	req := reEnrichRequest{
 		TrackName:   "Song",

lib/screens/track_metadata_edit_sheet.dart

@@ -643,6 +643,9 @@ class _EditMetadataSheetState extends State<_EditMetadataSheet> {
     if (currentTitle.isNotEmpty) {
       return titleMatches;
     }
+    if (currentAlbum.isNotEmpty) {
+      return albumMatches;
+    }
 
     return false;
   }