fix: handle missing electron executable gracefully

[jackwener]

Intent

Fix the Electron app launch path so a missing executable does not crash the CLI with an unhandled child-process error.

What changed

• add a guarded launchDetachedApp() helper in src/launcher.ts
• convert spawn ENOENT into a controlled CommandExecutionError with an actionable hint
• cover successful launch and ENOENT failure in src/launcher.test.ts

Verification

• npx vitest run src/launcher.test.ts src/electron-apps.test.ts src/engine.test.ts
• npm run typecheck
• npm run build
• node dist/main.js antigravity new now returns a controlled launch error instead of an unhandled crash