Confirm before installing updates

[enaboapps]

Summary

• add a native Electron warning dialog before installing downloaded updates
• default the confirmation to Cancel and return cancelled without installing when dismissed
• keep the existing silent install and forced relaunch path after confirmation
• add focused IPC tests for confirm, cancel, not-downloaded, and dialog copy behavior

Closes #220.

Validation

• npm test -- src/main/updates/update-ipc.test.ts src/main/updates/update-service.test.ts
• npm test
• npm run build