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/build.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+IMAGE_NAME="cadroute-builder-linux"
+
+echo "[cadroute] Building Docker image..."
+docker build -t "$IMAGE_NAME" "$SCRIPT_DIR"
+
+echo "[cadroute] Running Linux build..."
+docker run --rm \
+  --device /dev/fuse \
+  --cap-add SYS_ADMIN \
+  -v "$PROJECT_ROOT":/project \
+  -w /project \
+  -v cadroute-electron-cache:/root/.cache/electron \
+  -v cadroute-electronbuilder-cache:/root/.cache/electron-builder \
+  -v /etc/ssl/certs/ca-certificates.crt:/etc/ssl/certs/ca-certificates.crt:ro \
+  -e SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt \
+  -e NODE_EXTRA_CA_CERTS=/etc/ssl/certs/ca-certificates.crt \
+  "$IMAGE_NAME" \
+  bash -c "npm ci && npm run build -- --linux --x64"
+
+echo "[cadroute] Artefatti in: $PROJECT_ROOT/dist"