fix(layers): deleting a non-empty layer warns and removes its objects (#237)

Deleting a layer only dropped the layer record, leaving its objects in the
file still tagged with the (now-gone) layer name. Now:

- Empty layer → deleted straight away.
- Non-empty layer → a warning modal ("Layer X is not empty — N objects;
  deleting also removes them. Continue?") with Delete Objects / Cancel.
  Confirming erases every object on the layer, then removes the layer, as
  one undo step. The layer record is dropped before the erase so the
  locked-layer guard can't keep its objects.
- Layer "0" and the current layer can't be deleted (reported on the
  command line) instead of silently leaving a broken state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1370,6 +1370,7 @@ impl OpenCADStudio {
             SaveDialog => (560, 480),
             PointStyle => (360, 470),
             AttributeEditor => (640, 500),
+            LayerDeleteWarning => (440, 200),
         };
         Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
     }