desktop(architecture): complete typed preload bridge and stable Desktop IPC surface

[Whiteks1]

Summary

Turn the preload layer into a narrow, typed, stable Desktop bridge after the TypeScript base and main modularization are in place.

Includes

• typed preload API
• typed event subscriptions
• stable global Desktop surface exposed to the renderer
• removal of ad hoc typing gaps between main and renderer

Done when

• preload is fully typed
• renderer consumes a stable bridge contract
• no behavior change is introduced