fix: Keep the window draggable while a design opens (#643)

The loading box was a full-window modal whose backdrop covered the custom title bar. It is now a light overlay under the title bar, so dragging, snapping, resizing and the window buttons keep working during open, import and recovery. The timeline rebuild shows feature N of M, and Cancel or Esc stops it at the next feature (solidcraft_doc::progress). Recovery restore now runs off the window thread too. A control or MCP doc.open or import is answered once the file is in and doesn't block frames; edits sent meanwhile wait their turn. The web build still opens in place.

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

crates/ui-egui/src/scenario.rs

@@ -225,6 +225,12 @@ impl Harness {
             }
             self.frame();
         }
+        // A file opening or recovering in the background: wait for it (as a person would).
+        let began = std::time::Instant::now();
+        while !self.app.file_load.idle() && began.elapsed() < std::time::Duration::from_secs(120) {
+            self.frame();
+            std::thread::sleep(std::time::Duration::from_millis(1));
+        }
         self.frames(4);
     }