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/doc/src/lib.rs

@@ -32,6 +32,7 @@ pub mod naming;
 mod params;
 mod piece_names;
 mod profile_refs;
+pub mod progress;
 pub use profile_refs::{capture_profile_selection, profile_selection_indices};
 pub mod plastic;
 pub mod project;