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/buildArgs.ts

@@ -100,6 +100,10 @@ export interface AccessOptions {
   restrictFilenames: boolean
   /** absolute path to a --download-archive file; omit to disable archive tracking */
   downloadArchivePath?: string
+  /** YouTube extraction client override (e.g. 'web_safari'); empty/omitted = default */
+  youtubePlayerClient?: string
+  /** manually-supplied YouTube Proof-of-Origin token; empty/omitted = none */
+  youtubePoToken?: string
 }
 
 // Tuned for typical CDN-served video: 16 connections split across 16 segments,
@@ -278,6 +282,12 @@ function accessArgs(a: AccessOptions): string[] {
   else if (a.cookiesFile) args.push('--cookies', a.cookiesFile)
   if (a.restrictFilenames) args.push('--restrict-filenames')
   if (a.downloadArchivePath) args.push('--download-archive', a.downloadArchivePath)
+  // YouTube reliability: combine the player-client + PO-token overrides into one
+  // `youtube:` extractor-args group (semicolon-separated, as yt-dlp expects).
+  const yt: string[] = []
+  if (a.youtubePlayerClient?.trim()) yt.push(`player_client=${a.youtubePlayerClient.trim()}`)
+  if (a.youtubePoToken?.trim()) yt.push(`po_token=${a.youtubePoToken.trim()}`)
+  if (yt.length > 0) args.push('--extractor-args', `youtube:${yt.join(';')}`)
   return args
 }
 

src/main/download.ts

@@ -214,7 +214,9 @@ export function buildCommand(opts: StartDownloadOptions): string[] {
     cookiesFromBrowser: settings.cookieSource === 'browser' ? settings.cookiesBrowser : undefined,
     cookiesFile,
     restrictFilenames: settings.restrictFilenames,
-    downloadArchivePath: settings.downloadArchive ? getDownloadArchivePath() : undefined
+    downloadArchivePath: settings.downloadArchive ? getDownloadArchivePath() : undefined,
+    youtubePlayerClient: settings.youtubePlayerClient,
+    youtubePoToken: settings.youtubePoToken
   }
   const extraArgs = resolveExtraArgs(opts, settings)
   return buildArgs(opts, outputTemplate, options, getBinDir(), access, extraArgs)

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
     }