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
     }

src/renderer/src/components/SettingsView.tsx

@@ -226,6 +226,8 @@ export function SettingsView(): React.JSX.Element {
   const proxy = useSettings((s) => s.proxy)
   const rateLimit = useSettings((s) => s.rateLimit)
   const useAria2c = useSettings((s) => s.useAria2c)
+  const youtubePlayerClient = useSettings((s) => s.youtubePlayerClient)
+  const youtubePoToken = useSettings((s) => s.youtubePoToken)
   const cookieSource = useSettings((s) => s.cookieSource)
   const cookiesBrowser = useSettings((s) => s.cookiesBrowser)
   const restrictFilenames = useSettings((s) => s.restrictFilenames)
@@ -675,6 +677,28 @@ export function SettingsView(): React.JSX.Element {
             label={useAria2c ? 'On' : 'Off'}
           />
         </Field>
+
+        <Field
+          label="YouTube client (advanced)"
+          hint="Override the YouTube extraction client when downloads start failing the bot check. Try web_safari, tv, or mweb. Leave blank for yt-dlp's default."
+        >
+          <Input
+            value={youtubePlayerClient}
+            placeholder="web_safari"
+            onChange={(_, d) => update({ youtubePlayerClient: d.value })}
+          />
+        </Field>
+
+        <Field
+          label="YouTube PO token (advanced)"
+          hint="A manually-obtained Proof-of-Origin token for YouTube's bot check, sent via --extractor-args. Usually cookies (above) are the easier fix; automatic minting is planned."
+        >
+          <Input
+            value={youtubePoToken}
+            placeholder="(none)"
+            onChange={(_, d) => update({ youtubePoToken: d.value })}
+          />
+        </Field>
       </Card>
 
       <Card className={styles.card}>

src/renderer/src/main.tsx

@@ -27,6 +27,8 @@ if (import.meta.env.DEV && !window.api) {
     useAria2c: false,
     cookieSource: 'none',
     cookiesBrowser: 'chrome',
+    youtubePlayerClient: '',
+    youtubePoToken: '',
     restrictFilenames: false,
     downloadArchive: false,
     autoUpdateYtdlp: true,

src/renderer/src/store/settings.ts

@@ -21,6 +21,8 @@ const FALLBACK: Settings = {
   useAria2c: false,
   cookieSource: 'none',
   cookiesBrowser: 'chrome',
+  youtubePlayerClient: '',
+  youtubePoToken: '',
   restrictFilenames: false,
   downloadArchive: false,
   autoUpdateYtdlp: true,

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 */