feat(linux): bundle Python CLI and fix Electron packaged build

- contrib/linux/Dockerfile: add libpython3.11 + PyInstaller to build
  standalone cli binary; fix venv isolation via .dockerignore
- frontend/electron/main.cjs: use bundled cli binary in prod via
  process.resourcesPath, fallback to venv/python3 in dev
- frontend/package.json: add extraResources for cli binary,
  set output dir to release/
- frontend/vite.config.js: set base './' for file:// protocol in prod
- .dockerignore: exclude venv/, node_modules/, dist/, .git/
- .gitignore: ignore release/ output directories

.dockerignore

@@ -0,0 +1,10 @@
+venv/
+.venv/
+__pycache__/
+*.pyc
+node_modules/
+frontend/node_modules/
+frontend/dist/
+frontend/release/
+release/
+.git/