feat: add Opus 320kbps quality, remove Tidal HIGH tier

- Add YouTubeQualityOpus320 constant and opus_320 parser case in Go backend
- Expand opus supported bitrates to [128, 256, 320] across Go, Dart settings, and UI
- Update default YouTube Opus option from 256 to 320kbps
- Remove Tidal HIGH (lossy 320kbps) quality from Go backend, settings model,
  settings provider, download queue provider (both SAF and non-SAF paths),
  settings UI (quality option, format picker, helper methods), and l10n keys
- Add settings migration v6: auto-migrate users with audioQuality=HIGH to LOSSLESS
- Update and add Go test cases for opus_320 and adjusted max bitrate
- Regenerate l10n files, remove 10 unused downloadLossy* l10n keys

go_backend/tidal.go

@@ -1961,11 +1961,7 @@ func downloadFromTidal(req DownloadRequest) (TidalDownloadResult, error) {
 
 	outputExt := strings.TrimSpace(req.OutputExt)
 	if outputExt == "" {
-		if quality == "HIGH" {
-			outputExt = ".m4a"
-		} else {
-			outputExt = ".flac"
-		}
+		outputExt = ".flac"
 	} else if !strings.HasPrefix(outputExt, ".") {
 		outputExt = "." + outputExt
 	}
@@ -1979,7 +1975,7 @@ func downloadFromTidal(req DownloadRequest) (TidalDownloadResult, error) {
 		}
 		m4aPath = outputPath
 	} else {
-		if outputExt == ".m4a" || quality == "HIGH" {
+		if outputExt == ".m4a" {
 			filename = sanitizeFilename(filename) + ".m4a"
 			outputPath = filepath.Join(req.OutputDir, filename)
 			m4aPath = outputPath
@@ -1992,10 +1988,8 @@ func downloadFromTidal(req DownloadRequest) (TidalDownloadResult, error) {
 		if fileInfo, statErr := os.Stat(outputPath); statErr == nil && fileInfo.Size() > 0 {
 			return TidalDownloadResult{FilePath: "EXISTS:" + outputPath}, nil
 		}
-		if quality != "HIGH" {
-			if fileInfo, statErr := os.Stat(m4aPath); statErr == nil && fileInfo.Size() > 0 {
-				return TidalDownloadResult{FilePath: "EXISTS:" + m4aPath}, nil
-			}
+		if fileInfo, statErr := os.Stat(m4aPath); statErr == nil && fileInfo.Size() > 0 {
+			return TidalDownloadResult{FilePath: "EXISTS:" + m4aPath}, nil
 		}
 	}
 
@@ -2151,27 +2145,7 @@ func downloadFromTidal(req DownloadRequest) (TidalDownloadResult, error) {
 			fmt.Println("[Tidal] No lyrics available from parallel fetch")
 		}
 	} else if (isSafOutput && actualExt == ".m4a") || (!isSafOutput && strings.HasSuffix(actualOutputPath, ".m4a")) {
-		if quality == "HIGH" {
-			GoLog("[Tidal] HIGH quality M4A - skipping metadata embedding (file from server is already valid)\n")
-
-			if req.EmbedMetadata && req.EmbedLyrics && parallelResult != nil && parallelResult.LyricsLRC != "" {
-				lyricsMode := req.LyricsMode
-				if lyricsMode == "" {
-					lyricsMode = "embed"
-				}
-
-				if !isSafOutput && (lyricsMode == "external" || lyricsMode == "both") {
-					GoLog("[Tidal] Saving external LRC file for M4A (mode: %s)...\n", lyricsMode)
-					if lrcPath, lrcErr := SaveLRCFile(actualOutputPath, parallelResult.LyricsLRC); lrcErr != nil {
-						GoLog("[Tidal] Warning: failed to save LRC file: %v\n", lrcErr)
-					} else {
-						GoLog("[Tidal] LRC file saved: %s\n", lrcPath)
-					}
-				}
-			}
-		} else {
-			fmt.Println("[Tidal] Skipping metadata embedding for M4A file (will be handled after FFmpeg conversion)")
-		}
+		fmt.Println("[Tidal] Skipping metadata embedding for M4A file (will be handled after FFmpeg conversion)")
 	}
 
 	if !isSafOutput {
@@ -2181,10 +2155,6 @@ func downloadFromTidal(req DownloadRequest) (TidalDownloadResult, error) {
 	bitDepth := downloadInfo.BitDepth
 	sampleRate := downloadInfo.SampleRate
 	lyricsLRC := ""
-	if quality == "HIGH" {
-		bitDepth = 0
-		sampleRate = 44100
-	}
 	if req.EmbedMetadata && req.EmbedLyrics && parallelResult != nil && parallelResult.LyricsLRC != "" {
 		lyricsLRC = parallelResult.LyricsLRC
 	}

go_backend/youtube.go

@@ -29,6 +29,7 @@ var (
 type YouTubeQuality string
 
 const (
+	YouTubeQualityOpus320 YouTubeQuality = "opus_320"
 	YouTubeQualityOpus256 YouTubeQuality = "opus_256"
 	YouTubeQualityOpus128 YouTubeQuality = "opus_128"
 	YouTubeQualityMP3128  YouTubeQuality = "mp3_128"
@@ -37,7 +38,7 @@ const (
 )
 
 var (
-	youtubeOpusSupportedBitrates = []int{128, 256}
+	youtubeOpusSupportedBitrates = []int{128, 256, 320}
 	youtubeMp3SupportedBitrates  = []int{128, 256, 320}
 )
 
@@ -146,6 +147,8 @@ func parseYouTubeQualityInput(raw string) (format string, bitrate int, normalize
 	switch normalizedRaw {
 	case "opus_256", "opus256", "opus":
 		return "opus", 256, YouTubeQualityOpus256
+	case "opus_320", "opus320":
+		return "opus", 320, YouTubeQualityOpus320
 	case "opus_128", "opus128":
 		return "opus", 128, YouTubeQualityOpus128
 	case "mp3_320", "mp3320", "mp3", "":

go_backend/youtube_quality_test.go

@@ -30,8 +30,8 @@ func TestParseYouTubeQualityInput_Mp3NormalizesToSupportedBitrates(t *testing.T)
 
 func TestParseYouTubeQualityInput_PicksNearestSupportedBitrate(t *testing.T) {
 	_, opusBitrate, _ := parseYouTubeQualityInput("opus_999")
-	if opusBitrate != 256 {
-		t.Fatalf("expected opus normalization to 256, got %d", opusBitrate)
+	if opusBitrate != 320 {
+		t.Fatalf("expected opus normalization to 320, got %d", opusBitrate)
 	}
 
 	_, mp3Bitrate, _ := parseYouTubeQualityInput("mp3_1")
@@ -39,3 +39,16 @@ func TestParseYouTubeQualityInput_PicksNearestSupportedBitrate(t *testing.T) {
 		t.Fatalf("expected mp3 normalization to 128, got %d", mp3Bitrate)
 	}
 }
+
+func TestParseYouTubeQualityInput_Opus320(t *testing.T) {
+	format, bitrate, normalized := parseYouTubeQualityInput("opus_320")
+	if format != "opus" {
+		t.Fatalf("expected opus format, got %s", format)
+	}
+	if bitrate != 320 {
+		t.Fatalf("expected 320 bitrate, got %d", bitrate)
+	}
+	if normalized != YouTubeQualityOpus320 {
+		t.Fatalf("expected %s normalized, got %s", YouTubeQualityOpus320, normalized)
+	}
+}