Fix M18: make audio quality labels format-agnostic ("Best", not "Best (MP3)")

The audio quality preset "Best (MP3)" named a format that's wrong whenever the
audio format is opus/flac/wav. Renamed it to "Best" so the label never
contradicts the chosen audioFormat (which is picked separately). Bitrate presets
stay as bitrates and are already ignored for lossless formats (M21).

- AUDIO_QUALITY_OPTIONS[0]: 'Best (MP3)' -> 'Best' (buildArgs already had a
  `case 'Best'` returning '0', so arg generation is unchanged).
- Updated the main + renderer defaults and the preview/mock/test fixtures.
- Added a one-line migration in getSettings() that rewrites a stored legacy
  'Best (MP3)' to 'Best' so existing users' dropdowns match.

typecheck + 242 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/settings.ts

@@ -29,7 +29,7 @@ const DEFAULTS: Settings = {
   audioDir: '',
   defaultKind: 'video',
   defaultVideoQuality: 'Best available',
-  defaultAudioQuality: 'Best (MP3)',
+  defaultAudioQuality: 'Best',
   maxConcurrent: 2,
   filenameTemplate: '%(title)s.%(ext)s',
   // Follow the OS theme on first launch (SR1) so a dark-mode Windows user isn't
@@ -263,6 +263,11 @@ export function getSettings(): Settings {
   if (!(ACCENT_COLORS as readonly string[]).includes(cur.accentColor)) {
     s.set('accentColor', DEFAULTS.accentColor)
   }
+  // Migrate the legacy audio-quality label 'Best (MP3)' to the format-agnostic
+  // 'Best' so the dropdown matches and the label no longer names a format (M18).
+  if (cur.defaultAudioQuality === 'Best (MP3)') {
+    s.set('defaultAudioQuality', 'Best')
+  }
   // Hand callers (and, via IPC, the renderer) plaintext credentials — they're
   // only encrypted on disk (see withDecryptedSecrets / encryptSecret).
   cachedSettings = withDecryptedSecrets(s.store)