Complete Phase C (custom commands & yt-dlp updater) and Phase D (library, UX & notifications)

Phase C had been implemented in the working tree but never committed:
named custom-command templates (CRUD + per-download override), command
preview, and the in-app yt-dlp self-updater.

Phase D adds: history search/kind-filter/multi-select/bulk-delete/
re-download; native OS notifications on completion/failure; a private/
incognito download mode that skips history; settings+template backup/
restore via JSON; and a persistent error log surfaced as a Diagnostics
report in Settings.

See ROADMAP.md for the full per-item breakdown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/ytdlp.ts

@@ -1,7 +1,7 @@
 import { execFile } from 'child_process'
 import { existsSync } from 'fs'
 import { getYtdlpPath } from './binaries'
-import type { YtdlpVersionResult } from '@shared/ipc'
+import type { YtdlpVersionResult, YtdlpUpdateChannel, YtdlpUpdateResult } from '@shared/ipc'
 
 /**
  * Step-1 spike: spawn the bundled yt-dlp and read back `--version`.
@@ -30,3 +30,38 @@ export function getYtdlpVersion(): Promise<YtdlpVersionResult> {
     })
   })
 }
+
+/**
+ * Self-update the bundled yt-dlp.exe via its built-in `--update-to <channel>`
+ * (stable releases or the nightly build). Requires write access to the file
+ * yt-dlp.exe lives in, which holds for both the dev resources/bin checkout and
+ * a per-user install; it would fail under a locked-down system install.
+ */
+export function updateYtdlp(channel: YtdlpUpdateChannel): Promise<YtdlpUpdateResult> {
+  const ytdlpPath = getYtdlpPath()
+
+  if (!existsSync(ytdlpPath)) {
+    return Promise.resolve({
+      ok: false,
+      error: `yt-dlp.exe not found at ${ytdlpPath}\nDownload it into resources/bin/ (see the README there).`
+    })
+  }
+
+  return new Promise((resolve) => {
+    execFile(
+      ytdlpPath,
+      ['--update-to', channel],
+      { windowsHide: true, timeout: 60_000 },
+      (err, stdout, stderr) => {
+        if (err) {
+          const msg = (err as { killed?: boolean }).killed
+            ? 'Timed out updating yt-dlp.'
+            : (stderr || err.message).trim()
+          resolve({ ok: false, error: msg })
+          return
+        }
+        resolve({ ok: true, output: stdout.trim() })
+      }
+    )
+  })
+}