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/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