Incial

Add Windows desktop app (Electron shell) and README docs

Wraps the existing web app (login -> dashboard, same as browser) in a
native Electron window for Windows. Packaging via electron-builder is
wired up and tested end-to-end (produces a working NSIS installer).
The production URL still needs to be set in desktop/main.js before
building a real distributable installer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>