Add Seal-parity download options + playlist support (Phase A)

Phase A of the Seal feature-parity roadmap: configurable post-processing via a
shared DownloadOptions model (src/shared/ipc.ts), editable as persisted defaults
in Settings and overridable per-download in the download bar.

- audio formats (mp3/m4a/opus/flac/wav/aac), video container (mp4/mkv/webm),
  preferred-codec sort (-S vcodec)
- subtitles (download/embed/langs/auto), SponsorBlock (remove/mark + categories
  + force-keyframes-at-cuts), embed chapters/metadata/thumbnail, square-crop
  audio artwork
- playlist downloads: probe via `-J --flat-playlist`, inline selection UI
  (checkbox list, select-all/none, live count); each entry enqueued as its own
  single-video download, reusing the existing queue/concurrency/history
- reusable DownloadOptionsForm shared by Settings and the download bar so the
  defaults and per-download override never drift
- ROADMAP.md tracking full Seal parity (phases A-E)

Also includes in-tree security hardening that landed alongside: preload sandbox
(CJS preload output), http(s)-only window-open + blocked renderer navigation,
argv-injection guard for URLs (src/main/url.ts), extension-allowlisted file
open/reveal (src/main/reveal.ts), yt-dlp version-check timeout, and a minimal
window.electron presence marker.

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

src/main/ytdlp.ts

@@ -18,9 +18,12 @@ export function getYtdlpVersion(): Promise<YtdlpVersionResult> {
   }
 
   return new Promise((resolve) => {
-    execFile(ytdlpPath, ['--version'], { windowsHide: true }, (err, stdout, stderr) => {
+    execFile(ytdlpPath, ['--version'], { windowsHide: true, timeout: 15_000 }, (err, stdout, stderr) => {
       if (err) {
-        resolve({ ok: false, error: (stderr || err.message).trim() })
+        const msg = (err as { killed?: boolean }).killed
+          ? 'Timed out running yt-dlp.'
+          : (stderr || err.message).trim()
+        resolve({ ok: false, error: msg })
         return
       }
       resolve({ ok: true, version: stdout.trim() })