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/renderer/src/main.tsx

@@ -16,7 +16,7 @@ if (import.meta.env.DEV && !window.api) {
     audioDir: 'C:\\Users\\you\\Documents\\Audio',
     defaultKind: 'video',
     defaultVideoQuality: 'Best available',
-    defaultAudioQuality: 'Best (MP3)',
+    defaultAudioQuality: 'Best',
     maxConcurrent: 2,
     filenameTemplate: '%(title)s.%(ext)s',
     theme: 'system',

src/renderer/src/store/downloads.ts

@@ -221,7 +221,7 @@ const seed: DownloadItem[] = PREVIEW
         channel: 'ChillStudio',
         durationLabel: '1:00:21',
         kind: 'audio',
-        quality: 'Best (MP3)',
+        quality: 'Best',
         status: 'queued',
         progress: 0
       },

src/renderer/src/store/history.ts

@@ -25,7 +25,7 @@ const seed: HistoryEntry[] = PREVIEW
         channel: 'ChillStudio',
         url: 'https://youtube.com/watch?v=aaa1111',
         kind: 'audio',
-        quality: 'Best (MP3)',
+        quality: 'Best',
         sizeLabel: '142 MB',
         filePath: 'C:\\Users\\you\\Downloads\\Deep Focus — Ambient Mix.mp3',
         completedAt: Date.now() - 1000 * 60 * 60 * 26

src/renderer/src/store/settings.ts

@@ -10,7 +10,7 @@ const FALLBACK: Settings = {
   audioDir: PREVIEW ? 'C:\\Users\\you\\Documents\\Audio' : '',
   defaultKind: 'video',
   defaultVideoQuality: 'Best available',
-  defaultAudioQuality: 'Best (MP3)',
+  defaultAudioQuality: 'Best',
   maxConcurrent: 2,
   filenameTemplate: '%(title)s.%(ext)s',
   // Mirror main's DEFAULTS (SR1): follow the OS theme until real settings load,