Multiple windows in the same runtime

## Feature Request

Currently desktop always requires a new virtual dom with a different signal + context for every new window. It would be nice if we could allow the context to be the same so signals and globals work across the windows

## Implement Suggestion

Nico mentioned modifying the virtual dom to accept a foreign runtime might be enough which sounds feasible. Marking scopes as dirty with a raw message might cause some issues, but I think we almost exclusively use a wrapper in signals

Start by locating desktop window creation and the virtual DOM/runtime setup, then inspect how signals, globals, and scopes are propagated between windows. Check the existing signal wrapper and raw scope-dirty messaging before deciding whether foreign-runtime support is sufficient. Done means multiple windows can share one context, signals, and globals without requiring separate runtimes.

Written by the indexing model from the issue text.