Apply Linux no-sandbox launch flag

[shriram-svg]

Summary

• add --no-sandbox to Linux package launcher args
• append the same Electron command-line switch in the packaged Linux main process before ready
• avoid duplicating the switch when it is already present from launcher metadata

This covers both desktop launcher startup and direct AppImage/deb binary startup for Debian-based systems where Electron aborts on a misconfigured chrome-sandbox helper.

Closes #244.

Verification

• node -e electron-builder config validation for package.json#build
• npx npm@6 run lint
• npx npm@6 run test-api -- --runInBand
• ./node_modules/.bin/tsc --noEmit --pretty false
• ./node_modules/.bin/prettier --check package.json src/electron/index.ts
• git diff --check
• NODE_OPTIONS=--openssl-legacy-provider NODE_ENV=production BOOST_HUB_BASE_URL=https://boostnote.io ./node_modules/.bin/ts-node -P tsconfig-webpack.json scripts/build-electron-main.ts

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