fix: paper background no longer resizes when a viewport is added

White paper fill was calculated from entity bounding boxes (paper_entity_extents),
so adding a viewport caused the paper to visually shrink to the viewport's bounds.
Now paper_limits() (layout min/max limits) is used directly, and the unused
paper_entity_extents() function is removed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/properties.rs

@@ -312,7 +312,6 @@ impl H7CAD {
                 .add_entity_to_layout(entity, &layout)
             {
                 Ok(new_handle) => {
-                    // Auto-fit the new viewport to show model-space content.
                     self.tabs[i].scene.auto_fit_viewport(new_handle);
                 }
                 Err(e) => self