Fix desktop update restart flow

[kinwo]

Summary

• Route the update install action through an awaited main-process IPC handler so the app shuts down the CLI and then enters Electron’s quitAndInstall() restart path.
• Update the renderer banner to reflect the restart state while the install request is in flight.
• Add regression coverage for the restart banner flow and the update install IPC path.

Testing

• bun run verify passed.
• Added/updated Playwright coverage for the update banner restart flow in the desktop E2E suite.