Fix #830: the control channel's app.quit says when it asks first

Raw app.quit closed the window and answered null at once, so a client
couldn't tell that a save prompt was waiting. It now runs the menu's
Quit: {"pending": "saveChanges"} while a modified document is asked
about, as file.close and file.closeAll answer and the protocol
documentation says; otherwise null, and the window closes.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/ui-egui/src/control.rs

@@ -335,10 +335,9 @@ pub fn handle(app: &mut VectorcraftApp, ctx: &egui::Context, req: &ControlReques
             // No save dialog for an agent: the bytes come back, as in headless mode.
             None => wrap(app.run("document.export", p.clone())),
         },
-        "app.quit" => {
-            ctx.send_viewport_cmd(egui::ViewportCommand::Close);
-            ok(Value::Null)
-        }
+        // As the menu's Quit: `{"pending": "saveChanges"}` while a modified document is asked
+        // about; otherwise null, and the host closes the window (#830).
+        "app.quit" => wrap(app.run("app.quit", json!({}))),
         other => err(format!("unknown method `{other}`")),
     }
 }

crates/ui-egui/src/unsaved.rs

@@ -206,4 +206,23 @@ mod tests {
         app.run("app.quit", json!({})).unwrap();
         assert_eq!(app.ui.status, "quit");
     }
+
+    #[test]
+    fn the_control_channels_quit_says_when_it_asks_first() {
+        // #830: it answered null at once, before the question.
+        let (mut app, _) = app();
+        let ctx = egui::Context::default();
+        let quit = |app: &mut VectorcraftApp| {
+            let (req, _rx) = crate::control::ControlRequest::new("app.quit", json!({}));
+            let crate::control::Outcome::Done(r) = crate::control::handle(app, &ctx, &req) else { panic!("not done") };
+            r
+        };
+        new_doc(&mut app, true);
+        assert_eq!(quit(&mut app), json!({"ok": true, "result": {"pending": KIND}}));
+        assert_eq!((pending(&app), app.ui.status.as_str()), (Some(0), ""));
+        app.ui.dialog = None;
+        app.run("file.save", json!({"path": "/docs/saved.vectorcraft"})).unwrap();
+        assert_eq!(quit(&mut app), json!({"ok": true, "result": null}));
+        assert_eq!(app.ui.status, "quit");
+    }
 }