feat: per-viewport layer freeze columns in Layer Manager

Add VP freeze/thaw toggle columns to the layer panel when in a paper
space layout. Each viewport gets its own column (VP 1, VP 2, …) showing
a freeze icon per layer. Clicking toggles frozen_layers on the Viewport
entity and syncs back to the panel.

- scene: viewport_list() → [(handle, label, frozen_handles)]
- LayerPanel: VpCol + vp_frozen per layer, sync_with_viewports()
- Message::LayerToggleVpFreeze(layer_idx, vp_col_idx)
- Sync VP columns on layout switch, VPLAYER cmd, undo/redo, layer ops

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

src/app/history.rs

@@ -44,7 +44,8 @@ impl H7CAD {
         self.tabs[i].active_grip = None;
         self.tabs[i].dirty = snapshot.dirty;
         let doc_layers = self.tabs[i].scene.document.layers.clone();
-        self.tabs[i].layers.sync_from_doc(&doc_layers);
+        let vp_info = self.tabs[i].scene.viewport_list();
+        self.tabs[i].layers.sync_with_viewports(&doc_layers, vp_info);
         self.sync_ribbon_layers();
         self.refresh_properties();
     }