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>

test/updaterDownload.test.ts

@@ -33,16 +33,12 @@ class FakeResponse extends EventEmitter {
 }
 
 class FakeRequest extends EventEmitter {
-  headers: Record<string, string> = {}
   ended = false
   aborted = false
   redirectsFollowed = 0
   constructor(public url: string) {
     super()
   }
-  setHeader(k: string, v: string): void {
-    this.headers[k] = v
-  }
   end(): void {
     this.ended = true
   }
@@ -72,10 +68,6 @@ vi.mock('electron', () => ({
   shell: {}
 }))
 
-vi.mock('../src/main/settings', () => ({
-  getSettings: () => ({ updateToken: '' })
-}))
-
 import { downloadAppUpdate, cancelAppUpdate } from '../src/main/updater'
 import type { WebContents } from 'electron'