Phase 6: Auto-fit viewport and annotation scale hints

- Add model_space_extents() to Scene: collects key_vertices from all
  model-space entities to compute an AABB.
- Add auto_fit_viewport() to Scene: sets view_target to model centroid
  and derives custom_scale / view_height so the content fits the new
  viewport with a 10% margin.
- Call auto_fit_viewport() in commit_entity() after a viewport is added
  to a paper-space layout, so newly placed viewports immediately show
  model-space content at a sensible scale.
- Scale line_weight_px by the viewport scale factor when projecting
  model-space wires into paper space, so dimension arrows and annotation
  lines appear at the correct visual thickness.

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

src/app/properties.rs

@@ -220,7 +220,10 @@ impl H7CAD {
                 .document
                 .add_entity_to_layout(entity, &layout)
             {
-                Ok(_) => {}
+                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
                     .command_line
                     .push_error(&format!("Viewport could not be added: {e}")),