desktop(renderer): establish minimal React shell frame without migrating all surfaces

[Whiteks1]

Summary

Introduce the new React shell frame while keeping Electron, preload, IPC, and current Desktop behavior stable.

Includes

• shell layout frame
• topbar
• sidebar
• focused surface tabs
• support lane frame
• routing foundation for surfaces

Must not include

• broad surface migration
• legacy removal
• Vite-only experimentation outside the agreed shell scope

Done when

• the new shell frame exists and is usable
• current surfaces can still be hosted during transition
• no uncontrolled drift appears between old and new shell responsibilities