feat(editor): center on terrain (Home key), navigation improvements

- "Center on Terrain" (Home key or View menu): resets camera to center
  of loaded tile at 300 units altitude with 45-degree downward pitch.
  Essential for recovering when camera gets lost in empty space.
- Toast confirmation on center action

tools/editor/editor_ui.cpp

@@ -148,6 +148,7 @@ void EditorUI::renderMenuBar(EditorApp& app) {
             bool wf = app.isWireframe();
             if (ImGui::MenuItem("Wireframe", "F3", &wf)) app.setWireframe(wf);
             if (ImGui::MenuItem("Reset Camera")) app.resetCamera();
+            if (ImGui::MenuItem("Center on Terrain", "Home")) app.centerOnTerrain();
             ImGui::Separator();
             if (ImGui::BeginMenu("Sky / Lighting")) {
                 if (ImGui::MenuItem("Day")) app.setSkyPreset(0);