feat: Phase P reliability (YouTube extractor-args plumbing, signing + smoke-test docs)

- YouTube reliability: Settings.youtubePlayerClient + youtubePoToken ->
  one --extractor-args "youtube:player_client=...;po_token=..." group
  (accessArgs, unit-tested); advanced fields in Settings -> Network.
  Automatic WebView PO-token minting deferred (documented).
- docs/SMOKE-TEST.md: release-gate checklist for all shipped-but-untested
  OS wiring (cookies window, protocol/Send-to, scheduled sync, aria2c, proxy,
  tray/taskbar/pause-resume/terminal) — needs a human to run.
- docs/SIGNING.md: code-signing guide (CSC_LINK/CSC_KEY_PASSWORD, OV/EV,
  CI, HSM hook, verification). Build is signing-ready; needs a cert.
- i18n remains deferred (not faked).

typecheck + test (195) + build all clean. Roadmap: Phase P code shipped.

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

src/shared/ipc.ts

@@ -492,6 +492,17 @@ export interface Settings {
   cookieSource: CookieSource
   /** which browser to read cookies from when cookieSource is 'browser' */
   cookiesBrowser: CookieBrowser
+  /**
+   * YouTube reliability (Phase P). When set, emitted as
+   * `--extractor-args "youtube:player_client=…;po_token=…"`:
+   *  - playerClient: an alternate extraction client (e.g. 'web_safari', 'tv',
+   *    'mweb') — a common workaround for YouTube throttling/extraction breakage.
+   *  - poToken: a manually-supplied Proof-of-Origin token for the bot check.
+   * Both empty = yt-dlp's defaults. (Automatic WebView token minting is deferred;
+   * this is the argv plumbing it would feed.)
+   */
+  youtubePlayerClient: string
+  youtubePoToken: string
   /** sanitize output filenames to a portable ASCII-only subset (--restrict-filenames) */
   restrictFilenames: boolean
   /** record completed downloads in a yt-dlp --download-archive file to skip repeats */