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-tauri/tauri.conf.json

@@ -43,6 +43,11 @@
       "icons/128x128@2x.png",
       "icons/icon.icns",
       "icons/icon.ico"
+    ],
+    "externalBin": [
+      "binaries/yt-dlp",
+      "binaries/ffmpeg",
+      "binaries/ffprobe"
     ]
   }
 }