feat(build): aggiunge infrastruttura Docker per build Linux riproducibile

- contrib/linux: Dockerfile (node:20-bookworm-slim) e build.sh con mount CA certs
- package.json: target linux (AppImage/deb), files whitelist, electron in devDependencies
- main.js: icona finestra impostata esplicitamente per Linux
- build/.gitkeep: cartella build tracciata per icon.ico e icon.png

main.js

@@ -68,6 +68,7 @@ function createWindow() {
   const win = new BrowserWindow({
     width: 900,
     height: 640,
+    icon: path.join(__dirname, 'build', 'icon.png'),
     webPreferences: {
       preload: path.join(__dirname, 'preload.js'),
       contextIsolation: true,