feat: Layout Manager GUI (LAYOUTMANAGER / LAYOUTPANEL command)

Panel overlay showing all layouts with their active status.
Supports: select, rename, new, delete, reorder (◀▶), set current.
Also adds Scene::swap_layout_order() for tab-order resequencing.

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

src/app/commands.rs

@@ -1972,6 +1972,11 @@ impl H7CAD {
                 self.command_line.push_info("Opening Patreon page...");
             }
 
+            // ── Layout Manager GUI ─────────────────────────────────────────
+            "LAYOUTMANAGER"|"LAYOUTPANEL" => {
+                return Task::done(Message::LayoutManagerOpen);
+            }
+
             // ── Layout / viewport ──────────────────────────────────────────
             "MVIEW"|"MV" => {
                 if self.tabs[i].scene.current_layout == "Model" {