chore: remove dead code, fix error casing, and add lint rules

- Remove unused Go functions: buildRawAPEItem, loadCredentials,
  scanAudioFile, scanAudioFileWithKnownModTimeAndDisplayName,
  readM4AIndexValue, musixmatchSearchResponse/LyricsResponse structs
- Remove unused Go fields: downloadDir, utlsTransport.mu/h2Transports
- Lowercase Go error messages per convention (golint/ST1005)
- Simplify LyricsLine conversion and artistsMatch return
- Add Dart analysis rules: always_declare_return_types,
  avoid_types_as_parameter_names, strict_top_level_inference,
  type_annotate_public_apis
- Suppress SA1019 lint for required blowfish import

go_backend/progress.go

@@ -56,9 +56,6 @@ type progressBridgeState struct {
 }
 
 var (
-	downloadDir   string
-	downloadDirMu sync.RWMutex
-
 	multiProgress       = MultiProgress{Items: make(map[string]*ItemProgress)}
 	multiMu             sync.RWMutex
 	multiProgressDirty  = true
@@ -368,9 +365,6 @@ func ClearAllItemProgress() {
 }
 
 func setDownloadDir(path string) error {
-	downloadDirMu.Lock()
-	defer downloadDirMu.Unlock()
-	downloadDir = path
 	return nil
 }