Fix L62: drop hardcoded ext:'mp4' from the synthetic "Best available" format

The auto-best format option claimed ext:'mp4', but the real output container
for the best pick depends on the user's videoContainer setting (mp4/mkv/webm),
so it mislabeled mkv/webm outputs. ext is optional on FormatOption and the
option's label is the fixed "Best available", so the field is simply omitted.
hasAudio:true is kept (the best pick always merges an audio track).

typecheck + 242 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CODE-AUDIT.md

@@ -495,7 +495,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   hand-synced color constants (a "keep in sync" comment guards them).
 - [x] **L60 — `'external-url'` channel name lacks the `category:` prefix** every other IPC channel uses.
 - [x] **L61 — `taskbarProgress` is the lone `ipcMain.on`** (fire-and-forget) amid all-`invoke` channels.
-- [ ] **L62 — "Best available" synthetic format hardcodes `ext:'mp4'`/`hasAudio:true`** (probe.ts) —
+- [x] **L62 — "Best available" synthetic format hardcodes `ext:'mp4'`/`hasAudio:true`** (probe.ts) —
   mislabels when the chosen container is mkv/webm.
 - [x] **L63 — `audioQuality()` unknown label → silent `'0'` (best)** — the audio analog of H5.
 - [ ] **L64 — `ARIA2C_ARGS` connection params hardcoded** (`-x16 -s16 -k1M`), no user control.