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)

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,

src/shared/ipc.ts

@@ -128,7 +128,10 @@ export const VIDEO_QUALITY_OPTIONS = ['Best available', '1080p', '720p', '480p',
 export type VideoQuality = (typeof VIDEO_QUALITY_OPTIONS)[number]
 
 /** Quality/format labels for the audio download selector. */
-export const AUDIO_QUALITY_OPTIONS = ['Best (MP3)', '320 kbps', '192 kbps', '128 kbps'] as const
+// Format-agnostic: the audio container/codec is chosen separately (downloadOptions
+// .audioFormat); these are just the target quality. Bitrate values are ignored for
+// lossless formats (flac/wav) -- see buildArgs (M18/M21).
+export const AUDIO_QUALITY_OPTIONS = ['Best', '320 kbps', '192 kbps', '128 kbps'] as const
 export type AudioQuality = (typeof AUDIO_QUALITY_OPTIONS)[number]
 
 /** UI color theme: an explicit mode, or 'system' to follow the OS preference. */
@@ -409,7 +412,7 @@ export interface StartDownloadOptions {
   id: string
   url: string
   kind: MediaKind
-  /** one of the UI quality labels (e.g. '1080p', 'Best (MP3)') */
+  /** one of the UI quality labels (e.g. '1080p', 'Best') */
   quality: string
   /** absolute output directory; main falls back to the OS Downloads folder */
   outputDir?: string
@@ -614,7 +617,7 @@ export interface HistoryEntry {
   channel?: string
   url: string
   kind: MediaKind
-  /** Quality preset ('720p', 'Best available', 'Best (MP3)', …) or the probed
+  /** Quality preset ('720p', 'Best available', 'Best', …) or the probed
    *  format's display label. Used for display and, when no formatId is present,
    *  as the fallback quality selector on re-download (H5). */
   quality: string