Function run_simple 

pub fn run_simple<F>(gui: F) -> Result<(), DearAppError>
where
    F: FnMut(&Ui) + 'static,

Simple helper to run an app with a per-frame UI callback.

• Initializes Winit + WGPU + Dear ImGui
• Optionally initializes add-ons (ImPlot, ImNodes)
• Calls gui every frame with Ui and available add-ons