fix: stabilize desktop startup bundling

[xuewill]

Summary

• bundle internal @mango/* workspace packages into Electron main and preload outputs instead of externalizing them at runtime
• switch desktop startup to electron-vite's current ELECTRON_RENDERER_URL env and load the packaged renderer from out/renderer/index.html
• add smoke and unit coverage so stale workspace imports and deprecated MAIN_WINDOW_VITE_* globals fail fast

Test Plan

• pnpm verify
• pnpm --filter @mango/desktop dev
• subagent code review: no findings