feat(io): show loading overlay while opening CAD files

Big DWG/DXF files block the UI for seconds during parse + cache build,
leaving the user wondering whether the app froze. Split the file-open
flow into pick → load and show a modal overlay during the load: file
name, size, phase label (Reading / Parsing / Building caches /
Finalizing), animated indeterminate bar, and Cancel button.

Phase is tracked via a shared AtomicU8 written by the loader thread
and read by the overlay each frame. Cancel hides the overlay and
discards the result; the parser thread runs to completion in the
background since acadrust has no interrupt hook.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/view.rs

@@ -577,6 +577,12 @@ impl H7CAD {
             }
         };
 
+        let open_progress_layer: Element<'_, Message> = if let Some(p) = &self.opening {
+            crate::ui::open_progress::view(p, iced::time::Instant::now())
+        } else {
+            iced::widget::Space::new().width(0).height(0).into()
+        };
+
         stack![
             main_ui,
             self.app_menu.view(),
@@ -584,7 +590,8 @@ impl H7CAD {
             scale_layer,
             dropdown_layer,
             layout_ctx_layer,
-            viewport_ctx_layer
+            viewport_ctx_layer,
+            open_progress_layer,
         ]
         .into()
     }