fix(widgets): align tabbed post layouts with reference

tests/widget-system/a11y-contract.test.cjs

@@ -14,7 +14,9 @@ const general = read('src/WidgetSystem/Renderers/GeneralRenderer.php');
 
 for (const key of ['ArrowRight', 'ArrowLeft', 'Home', 'End']) assert.match(js, new RegExp(key));
 assert.match(js, /setAttribute\('tabindex', tab === button \? '0' : '-1'\)/);
-assert.match(js, /setAttribute\('aria-labelledby', button\.id\)/);
+// Each pane receives its label in server markup. Switching activates that
+// existing aria-controls target instead of relabelling the inner AJAX pager.
+assert.match(js, /item\.id === button\.getAttribute\('aria-controls'\)/);
 assert.match(content, /role="tab"[^;]+aria-controls=/);
 assert.match(content, /role="tabpanel"[^;]+aria-labelledby=/);
 assert.match(knowledge, /role="tab"[^;]+aria-controls=/);