chore: license the project under MIT

Replace the placeholder "UNLICENSED" with the MIT License so the now-public
repo is genuinely free and open source. Adds a LICENSE file and documents the
app-vs-bundled-binary licensing split in the README (yt-dlp/FFmpeg/aria2 are
invoked as separate executables, so they remain under their own licenses).

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

package.json

@@ -4,7 +4,7 @@
   "description": "A yt-dlp frontend for Windows",
   "main": "./out/main/index.js",
   "author": "AeroFetch",
-  "license": "UNLICENSED",
+  "license": "MIT",
   "homepage": "https://gitea.netbird.zimspace.uk:5938/debont80/AeroFetch",
   "repository": {
     "type": "git",