feat(desktop): add optional Electron desktop shell for Windows

[mochen555]

Summary

Adds an optional, self-contained Electron desktop shell (desktop/) for
StaffDeck on Windows. It reuses the existing PyInstaller-packed backend
(packaging/out/staffdeck) and loads the already-served web UI into a native
window instead of the system browser.

Additive & opt-in: existing backend/frontend/official pipeline untouched.

Features

• Auto-launches the backend (dev: backend/.venv, packaged: resources/backend/staffdeck.exe)
• Backend runs headless — no extra browser tab
• Native window loads /chat/
• System tray: close-to-tray (stay resident), tray menu (open / restart / quit),
  click-to-toggle window
• electron-builder bundles backend + tray icon

Packaging: problems encountered & how they were solved

• Electron binary / NSIS download in CN network
  Added CN mirrors in desktop/.npmrc and env vars (ELECTRON_MIRROR,
  ELECTRON_BUILDER_BINARIES_MIRROR) to speed up downloads.
• electron-builder winCodeSign symlink permission error on Windows
  The bundled winCodeSign archive contains macOS .dylib symlinks; extracting
  them fails with "client does not possess the required privilege" unless
  Windows Developer Mode is enabled. Workaround: enable Developer Mode, or
  run as admin. This is a known electron-builder quirk on non-admin Windows.
• PyInstaller backend must be built BEFORE electron-builder
  electron-builder.yml extraResources.from points at packaging/out/staffdeck,
  so run the official backend build first (build_windows.ps1 / the ultrarag.spec
  command) — otherwise the installer has no backend to bundle.
• Packaged app opens an extra browser
  desktop_launcher.py auto-opens the browser unless STAFFDECK_HEADLESS=1;
  the shell sets this env var when spawning the backend.

Build & run

# dev
cd desktop && npm install && npm start

# backend (first)
$env:VERSION="0.5.0"; .\packaging\build_windows.ps1   # or run ultrarag.spec
# installer
cd desktop && npm run dist    # => dist/StaffDeck Setup 0.5.0.exe