Module context 

AppContext — per-frame handle handed to super::App::update.

Wraps the backend’s per-tick context (egui’s Context, in practice) behind a neutral surface. Apps use it to:

• install / swap the UiTheme and FontPack at startup;
• request a repaint from a background thread or async event;
• query the screen size for responsive layouts;
• hand a callback the central paint region (which receives a &mut dyn UiBuilder — same trait the editor’s panels use).

Traits

AppContext

Top-level app context — one per frame.