chore: scaffold desktop shell

[rogerchappel]

Summary

• add an isolated apps/desktop Electron package with app-local lockfile
• load CrewCmd from CREWCMD_DESKTOP_SERVER_URL or http://localhost:3000
• constrain desktop navigation to HTTPS or localhost CrewCmd URLs and open external URLs in the system browser

Scope

• no root package.json, root lockfile, workspace manifest, or CI changes
• no preload bridge, secure storage, notifications, tray, updater, signing, or OpenClaw runtime bundling

Verification

• npm view electron version -> 41.4.0
• git diff --check
• node --check apps/desktop/src/main.cjs
• cd apps/desktop && pnpm install --frozen-lockfile
• pnpm exec tsc --noEmit --pretty false
• push hook: pnpm typecheck
• push hook: pnpm build

Risk

Medium. This introduces an isolated Electron app package and local lockfile, but does not affect root installs or web app runtime behavior.