Start card: every sample as a card with its thumbnail, flagship first, scrolling (#245)

The Start card and File › Samples show a grid of sample cards (thumbnail, name,
one-line description, the full text on hover) in a scroll area with a solid
scrollbar; Help › Samples keeps rows in the same order. The card stays clear of
the view cube and the navigation bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/appbar.rs

@@ -330,7 +330,7 @@ fn help_menu(app: &mut ModelApp, ui: &mut egui::Ui) {
     ui.set_max_width(340.0);
     crate::frame::menu_heading(ui, &t, "SAMPLES");
     let mut open = None;
-    crate::files::sample_rows(ui, &t, &mut open);
+    crate::samples::rows(ui, &t, &mut open);
     ui.separator();
     crate::frame::menu_heading(ui, &t, "NAVIGATION");
     ui.label("Left drag orbits (or pans or zooms: the navigation bar), right or middle drag pans, the wheel zooms, F or a double click fits.");