feat: bundle yt-dlp and ffmpeg as sidecars

The app is now self-contained: yt-dlp, ffmpeg and ffprobe ship inside
the bundle and are resolved beside the executable, so a fresh Mac needs
nothing installed. A system copy still wins when present, which is how
to run a newer yt-dlp than the bundled one. yt-dlp is pointed at the
bundled ffmpeg explicitly, since a bundled app has no reason to have one
on PATH.

The binaries stay out of git (~140MB); scripts/fetch-binaries.sh pulls
them. Homebrew's ffmpeg cannot be used — it links a dozen dylibs and
does not relocate — so the static arm64 build is used instead.

Version checks moved to the async Command. yt-dlp is a PyInstaller
bundle that unpacks ~37MB on first run, and the blocking call was
stalling a runtime worker for roughly twenty seconds.

Bundle size goes from 19MB to 153MB.

src/components/Settings.tsx

@@ -216,14 +216,19 @@ export default function Settings({
                 </div>
               </div>
 
+              <p className={`mt-2 ${HELP}`}>
+                yt-dlp and ffmpeg ship inside the app, so nothing needs installing. A copy
+                on your system is used instead if one is present, which is how you can run
+                a newer yt-dlp than the bundled one.
+              </p>
+
               {missing && (
                 <div className="mt-2 rounded-lg border border-red-500/30 bg-red-500/5 p-3">
                   <p className="text-[11px] leading-snug text-red-700 dark:text-red-300">
-                    Downloads need both tools. Install them with:
+                    A bundled tool is missing, which should not happen. Reinstalling the app
+                    will restore it; meanwhile <code>brew install yt-dlp ffmpeg</code> also
+                    works.
                   </p>
-                  <code className="mt-1.5 block rounded bg-slate-100 px-2 py-1 text-[11px] dark:bg-slate-800">
-                    brew install yt-dlp ffmpeg
-                  </code>
                 </div>
               )}
             </section>