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

contrib/linux/Dockerfile

@@ -0,0 +1,12 @@
+FROM node:20-bookworm-slim
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libudev-dev \
+    libusb-1.0-0-dev \
+    fuse \
+    rpm \
+    dpkg-dev \
+    python3 \
+    make \
+    g++ \
+ && rm -rf /var/lib/apt/lists/*
+WORKDIR /project