Updates test: answer the startup prompt before clicking in Preferences

preferences_turn_the_check_at_start_off starts with the check at start on. That check answers on
its own thread, and when its prompt opens between frames it takes the click meant for the
Preferences check box, so the setting stays on. Windows runners hit it often. The test now waits
for the prompt and answers Later first, like the prompt test beside it, through one shared helper.

crates/ui-egui/tests/it/updates.rs

@@ -169,6 +169,8 @@ fn turning_the_check_at_start_on_waits_for_the_next_start() {
 #[test]
 fn preferences_turn_the_check_at_start_off() {
     let (mut h, _calls) = started_with(r#"{"update_check_at_start": true}"#);
+    // The check at start answers on its own thread; its prompt would take the click below.
+    later_on_the_startup_prompt(&mut h);
     h.state_mut().execute("app.preferences");
     h.run_steps(3);
     h.get_by_label("Check for updates when PdfCraft starts").click();
@@ -196,6 +198,12 @@ fn the_check_at_start_offers_the_prompt_and_later_dismisses_it() {
         app.set_startup_update_check(true);
         app
     });
+    later_on_the_startup_prompt(&mut h);
+    assert!(h.query_by_label_contains("is available").is_none(), "Later closes the prompt");
+}
+
+/// Waits for the check at start to offer its prompt, then answers Later.
+fn later_on_the_startup_prompt(h: &mut Harness<'static, PdfCraftApp>) {
     let deadline = std::time::Instant::now() + std::time::Duration::from_secs(30);
     while h.query_by_label_contains("is available").is_none() {
         h.run_steps(2);
@@ -204,5 +212,4 @@ fn the_check_at_start_offers_the_prompt_and_later_dismisses_it() {
     }
     h.get_by_label("LATER").click();
     h.run_steps(3);
-    assert!(h.query_by_label_contains("is available").is_none(), "Later closes the prompt");
 }