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

analysis_options.yaml

@@ -36,7 +36,11 @@ linter:
   rules:
     # avoid_print: false  # Uncomment to disable the `avoid_print` rule
     # prefer_single_quotes: true  # Uncomment to enable the `prefer_single_quotes` rule
+    always_declare_return_types: true
     avoid_dynamic_calls: true
+    avoid_types_as_parameter_names: true
+    strict_top_level_inference: true
+    type_annotate_public_apis: true
     cancel_subscriptions: true
     close_sinks: true