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>

test/real-download.integration.test.ts

@@ -146,7 +146,7 @@ describe.skipIf(!RUN)('real yt-dlp downloads (buildArgs end-to-end)', () => {
       id: 'crop',
       url: 'https://www.youtube.com/watch?v=jNQXAC9IVRw', // "Me at the zoo", 19s
       kind: 'audio',
-      quality: 'Best (MP3)'
+      quality: 'Best'
     }
     const options = {
       ...DEFAULT_DOWNLOAD_OPTIONS,
@@ -181,7 +181,7 @@ describe.skipIf(!RUN)('real yt-dlp downloads (buildArgs end-to-end)', () => {
       id: 'sb',
       url,
       kind: 'audio',
-      quality: 'Best (MP3)'
+      quality: 'Best'
     }
     const options = {
       ...DEFAULT_DOWNLOAD_OPTIONS,