desktop(cleanup): retire legacy shell renderer after new Desktop migration slices land

[Whiteks1]

Summary

Remove the transitional legacy shell once the new architecture owns the Desktop experience end to end.

Includes

• retire obsolete renderer paths
• remove transitional glue no longer needed
• keep Electron, preload, and Desktop runtime validation intact

Done when

• legacy shell code is no longer the active Desktop path
• transitional duplication is removed
• the new Desktop architecture is the only maintained path