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>

CODE-AUDIT.md

@@ -274,7 +274,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   persisted data lives in main, so nothing is lost).*
 - [x] **M17 — `statusByUrl` collapses duplicate URLs.** LibraryView maps URL→status into one
   `Map`; with "Download anyway" duplicates, a row can reflect the wrong item's state.
-- [ ] **M18 — Audio "quality" presets hard-code MP3.** `QUALITY_OPTIONS.audio` labels ("Best
+- [x] **M18 — Audio "quality" presets hard-code MP3.** `QUALITY_OPTIONS.audio` labels ("Best
   (MP3)", "320 kbps") are shown even when `downloadOptions.audioFormat` is opus/flac/wav, so the
   label contradicts the actual output format; `--audio-quality` is also meaningless for lossless.
 - [x] **M19 — Sidebar-collapsed pref uses `localStorage`** while every other preference uses