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/main/settings.ts

@@ -36,6 +36,8 @@ const DEFAULTS: Settings = {
   useAria2c: false,
   cookieSource: 'none',
   cookiesBrowser: 'chrome',
+  youtubePlayerClient: '',
+  youtubePoToken: '',
   restrictFilenames: false,
   downloadArchive: false,
   // yt-dlp is self-managed: a writable copy under userData, auto-updated on the
@@ -256,6 +258,8 @@ export function setSettings(partial: Partial<Settings>): Settings {
       // and exported by exportBackup as-is — documented, not masked.
       case 'proxy':
       case 'rateLimit':
+      case 'youtubePlayerClient':
+      case 'youtubePoToken':
         if (typeof value === 'string') s.set(key, value)
         break
     }