feat: Implement IPC update channel with Mountain UpdateService

[NikolaRHristov]

Context

WindServiceHandlers.rs stubs the update channel (7 methods: checkForUpdates, downloadUpdate, applyUpdate, quitAndInstall, isLatestVersion, setInternalOrg, _getInitialState).

Currently returns static stub dict. Mountain doesn't implement IUpdateService yet.

Expected

• Auto-update check against release feed
• Download binary delta
• Apply and restart

Priority: Low (not blocking workbench)