Add mobile::dialog_width(ctx, desired) — a min()-clamp that's a no-op on wide
desktop screens. Clamp the Export and Export Progress modals and the Sample
Import window to it. Render Preferences as an egui::Modal (dim backdrop, centered,
no title bar) with a screen-fit scroll height when mobile_active(); desktop keeps
its window. Extracted the preferences body into render_body() shared by both paths.
Skyler Lehmkuhl