Initial commit: MusicDownload con UI pywebview

- Frontend HTML/CSS/JS in webui/ (stile Spotify-like, hero gradient, modal blur)
- API bridge JS<->Python in api/bridge.py
- Core invariato: downloader yt-dlp, upgrader, spotify client, config
- Build scripts macOS (WebKit) e Windows (EdgeChromium)
- .gitignore esclude credenziali (config.json, cookies.txt) e artefatti build

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

config.example.json

@@ -0,0 +1,9 @@
+{
+  "client_id": "",
+  "client_secret": "",
+  "bitrate": "320K",
+  "hq_threshold": 310,
+  "cookies_path": "",
+  "output_dir": "",
+  "theme": "dark"
+}