refactor: clear remaining lint errors (comma operator, bind:this ref)

- splitArray: replace the comma-operator reduce body with an explicit
  block + return (no-sequences); behaviour unchanged
- BreadcrumbHeaderSeeded: declare the bind:this ref with $state() so it
  is not flagged as never-assigned (oxlint cannot see template bindings),
  matching the rest of the codebase; guard the onMount use

src/features/Breadcrumb/ui/BreadcrumbHeader/BreadcrumbHeaderSeeded.svelte

@@ -11,10 +11,14 @@ const sections = [
     { index: 102, title: 'Spacing' },
 ];
 
-/** @type {HTMLDivElement} */
-let container;
+/** @type {HTMLDivElement | undefined} */
+let container = $state();
 
 onMount(() => {
+    if (!container) {
+        return;
+    }
+
     for (const section of sections) {
         const el = /** @type {HTMLElement} */ (container.querySelector(`[data-story-index="${section.index}"]`));
         scrollBreadcrumbsStore.add({ index: section.index, title: section.title, element: el }, 96);