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/cmd_result.rs

@@ -348,6 +348,10 @@ impl H7CAD {
                     if thawed_count > 0 {
                         self.command_line.push_info(&format!("VPLAYER: {thawed_count} layer(s) thawed in viewport."));
                     }
+                    // Sync layer panel so VP freeze columns update immediately.
+                    let doc_layers = self.tabs[i].scene.document.layers.clone();
+                    let vp_info = self.tabs[i].scene.viewport_list();
+                    self.tabs[i].layers.sync_with_viewports(&doc_layers, vp_info);
                 }
 
                 // Show updated prompt (command stays active for more operations).