fix: Keep the loading card's Cancel button still; deterministic Cancel test (#686)

The loading card was centred on its height, so Cancel moved when the progress bar or the saved preview appeared, and a click could miss it. That's what made the Cancel test flaky on slow runners. The card is now anchored at its top, Cancel sits in the heading row and the progress bar is always shown. Progress::hold_at lets the test hold a load at a feature boundary until it's cancelled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/file_load_ui_tests.rs

@@ -154,8 +154,17 @@ fn cancel_stops_the_open_and_keeps_the_current_design() {
     let mut w = Win::new();
     w.app.run("solid.box", json!({"length": 10, "width": 10, "height": 10})).unwrap();
     let current = w.app.session.doc.clone();
+    // The worker waits before the third feature until Cancel: the load is certainly still
+    // running when Cancel is clicked, on a fast machine or a slow one.
+    w.app.file_load.hold_at = Some(2);
     w.app.open_path_async(&path);
-    // The overlay is laid out on its first frame and shown from the next.
+    let began = std::time::Instant::now();
+    while w.app.file_load.features().is_none_or(|(done, _)| done < 2) {
+        assert!(began.elapsed() < std::time::Duration::from_secs(120), "the load reached the held feature");
+        w.frame(vec![]);
+        std::thread::sleep(std::time::Duration::from_millis(1));
+    }
+    // The overlay (and any saved preview the worker sent) is laid out and shown.
     for _ in 0..3 {
         w.frame(vec![]);
     }