PromptBox3D and CameraSettingsModal now accept plain values + setters
instead of `Signal<T>` props. The camera/focalLengthDragging/
isPromptBoxFocused signals are gone — store is the single source of
truth. Engine code (editor/save_manager/scene) reads/writes via
`usePageSceneStore.getState()` and store actions. Demo asset signals
are now plain constants (they were never reactive).
Tanish Manku
28db87c1b2