Fix OK lost while a joint animates; Browser Joints folder inside the root component

Clicking OK during the Joint dialog's motion animation was lost about half the time: the
preview's busy spinner came and went in the dialog footer between frames, shifting the
automatic ids of OK and Cancel, so a click pressed in one frame and released in the next landed
on two different ids. The spinner's place is now always taken. Reproduced through the control
channel (6 of 14 OK clicks at different animation phases lost before, 0 of 14 after) and
covered by a test that presses OK with the preview busy and releases it once idle.

The Joints folder is drawn by component_rows for the root, after its folders (as agreed with
solvecraft-shell).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/ui-egui/src/dialogs.rs

@@ -1386,8 +1386,13 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                 ui.painter().circle_stroke(r.center(), 7.0, Stroke::new(1.2, t.text_dim));
                 ui.painter().text(r.center(), egui::Align2::CENTER_CENTER, "i", egui::FontId::proportional(11.0), t.text_dim);
                 resp.on_hover_text(heading.as_str());
+                // The spinner's place is always taken: a widget that comes and goes would shift
+                // the automatic ids of OK and Cancel, and a click pressed in one frame and
+                // released in the next would be lost (an animating preview toggles it often).
+                let (sr, sresp) = ui.allocate_exact_size(vec2(14.0, 14.0), egui::Sense::hover());
                 if app.preview.busy {
-                    ui.add(egui::Spinner::new().size(14.0)).on_hover_text("Updating the preview");
+                    egui::Spinner::new().size(14.0).paint_at(ui, sr);
+                    sresp.on_hover_text("Updating the preview");
                 }
                 ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
                     let close = if matches!(d.kind, Kind::Params { .. } | Kind::Measure { .. } | Kind::Preferences) { "Close" } else { "Cancel" };