Tab: fix aria-controls

When porting, I thought I was being clever by just using ids instead of refs in $api.panels
But I didn't notice that the refs let you distinguish those panels which are rendered from those which are not.
My implementation had aria-controls= set for all tabs, even when their panels were not in the DOM.

src/lib/components/tabs/Tab.svelte

@@ -87,11 +87,12 @@
     $api.setSelectedIndex(myIndex);
   }
 
+  $: myPanelRef = $api.panels[myIndex]?.ref;
   $: propsWeControl = {
     id,
     role: "tab",
     type: resolveButtonType({ type: $$props.type, as }, tabRef),
-    "aria-controls": $api.panels[myIndex],
+    "aria-controls": $myPanelRef ? $api.panels[myIndex]?.id : undefined,
     "aria-selected": selected,
     tabIndex: selected ? 0 : -1,
     disabled: disabled ? true : undefined,