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/settings.ts

@@ -62,12 +62,12 @@ function getStore(): Store<Settings> {
 let cachedSettings: Settings | null = null
 
 // --- Credential encryption at rest ------------------------------------------
-// proxy / youtubePoToken / updateToken can carry secrets (a proxy password, API
-// tokens). They're stored encrypted via Electron safeStorage (DPAPI on Windows)
-// so a leaked settings.json doesn't expose them. They're decrypted before
-// reaching the renderer; backup export strips them entirely (see backup.ts, M22).
+// proxy / youtubePoToken can carry secrets (a proxy password, API tokens).
+// They're stored encrypted via Electron safeStorage (DPAPI on Windows) so a
+// leaked settings.json doesn't expose them. They're decrypted before reaching
+// the renderer; backup export strips them entirely (see backup.ts, M22).
 // Exported so backup.ts shares this one list rather than keeping a parallel copy.
-export const SECRET_KEYS = ['proxy', 'youtubePoToken', 'updateToken'] as const
+export const SECRET_KEYS = ['proxy', 'youtubePoToken'] as const
 
 // Marks a value produced by encryptSecret, so a read can tell ciphertext from a
 // legacy plaintext value (written before encryption existed, or while safeStorage