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.

.gitignore

@@ -27,3 +27,6 @@ dist-ssr
 *.njsproj
 *.sln
 *.sw?
+
+# Bundled helper binaries — ~140MB, fetched by scripts/fetch-binaries.sh
+src-tauri/binaries/