feat(desktop): add browser-mode IPC shim for full UI without Electron

In-memory implementation of window.codesign so the renderer runs standalone
via vite.browser.config.ts with design CRUD, chat, snapshots, and comments
all functional. Generation returns a placeholder HTML card.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tiegebentley