feat(setup): fetch ffmpeg on first run instead of bundling it

Ship a smaller installer that no longer carries ffmpeg.exe/ffprobe.exe
(the bulk of its size). On first run they are downloaded from a pinned
upstream archive (gyan 8.1.2), verified against a pinned SHA-256, and
unpacked with the OS tar.exe into the managed userData/bin dir -- the
same place as the self-updating yt-dlp, so they survive app updates and
portable re-extraction. A hard onboarding gate blocks the app until they
are present, with a "locate existing ffmpeg" folder-picker fallback for
offline machines and a Repair action in Settings > About.

The updater's streaming/checksum/redirect/idle-timeout download loop is
extracted to lib/verifiedDownload.streamVerifiedFile and shared by both
the app-installer download and the ffmpeg fetch; the updater's public
behaviour and error strings are unchanged (its boundary tests still pass).
No new npm dependency: extraction uses the System32 bsdtar resolved by
absolute path (audit F3).

Note: this commit also carries pre-existing, in-progress aria2c/network
and updater-token work that was already uncommitted in the working tree
and is entangled with the above in shared files (updater.ts, ipc.ts,
preload/index.ts, mockApi.ts, shared/ipc.ts, plus the settings/network
files and aria2c.exe). It was not cleanly separable by path, so it is
included here rather than split out.

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

src/main/core/buildArgs.ts

@@ -4,8 +4,9 @@
  * This module is deliberately free of any electron / node-runtime dependency
  * (it only imports types + the BEST_FORMAT_ID const from the shared contract)
  * so the argv it produces can be unit-tested without spinning up Electron.
- * `buildArgs` takes the ffmpeg/yt-dlp `binDir` as a parameter rather than
- * resolving it itself; the caller in download.ts passes `getBinDir()`.
+ * `buildArgs` takes the `binDir` as a parameter (used only for `--ffmpeg-location`)
+ * rather than resolving it itself; download.ts passes `getFfmpegDir()` — the managed
+ * userData/bin where the first-run download installs ffmpeg/ffprobe.
  */
 
 import { join } from 'path'