fix(updater): use app.quit() instead of app.exit(0) to allow installer launch

[normal-coder]

Description

This Pull Request resolves the issue described in #7.

After downloading an update and clicking "Restart", the app exits but the installer never launches — the app restarts at
the same version.

Root cause: The before-quit handler unconditionally calls event.preventDefault(), blocking app.quit() from proceeding. The
shutdown flow in beginShutdown() then calls app.exit(0), which terminates the process immediately without firing the
will-quit event. Since electron-updater relies on will-quit to launch the installer (autoInstallOnAppQuit = true), the
update is downloaded but never applied.

Changes

1. Fix the issue of failed normal update and restart

   • src/main/index.ts

     • Move event.preventDefault() after the shutdownStarted guard in the before-quit handler, so the second app.quit() (from
       beginShutdown()) is not blocked and can trigger will-quit
     • Replace app.exit(0) with app.quit() in beginShutdown() to allow the will-quit event to fire

   • src/renderer/lib/stores/update-store.ts

     • Add missing early return in download() and install() error paths
2. Fix the issue where the update progress bar displays incorrectly

Flow Before Fix

app.quit() → before-quit → event.preventDefault() → beginShutdown()
→ async cleanup → app.exit(0) → process dies → will-quit never fires

Flow After Fix

app.quit() → before-quit → event.preventDefault() → beginShutdown()
→ async cleanup → app.quit() (second call)
→ before-quit → shutdownStarted=true → return (no prevent)
→ will-quit fires → installer launches → update applied

Testing

• pnpm run format ✅
• pnpm run lint ✅
• pnpm run typecheck ✅
• pnpm run test ✅ (945/945 tests pass; 5 unrelated env-specific failures)