Keep Open and Save asynchronous to prevent macOS resize crashes (#1127)

Open and Save use parented AsyncFileDialog sheets; their answers run on the UI thread in a later frame, so resizing the window while a picker is open no longer re-enters winit and aborts. Editors, tab changes, Close/Quit and model-changing control requests wait for the answer; the local library Open and Export in the Appearance dialog use the same flow. Fixes #1088.

crates/ui-egui/src/control.rs

@@ -221,6 +221,7 @@ fn dispatch(app: &mut SolveApp, ctx: &egui::Context, method: &str, p: &Value) ->
             "renderer": if app.viewport.gpu.is_some() { "gpu" } else { "cpu" },
             "frame_ms": app.frame_ms,
             "file_busy": app.file_load.busy(),
+            "file_picker_busy": app.file_picker_busy(),
             "build_ms": app.viewport.build_ms,
             "preview": {"active": app.preview.active, "busy": app.preview.busy, "error": app.preview.error, "ms": app.preview.ms, "replaced": app.preview.replaced},
             "home": app.home.open,