docs(plan): Task 13 — match the type-safety fixes applied in Task 12

page.params.id is string|undefined in SvelteKit types; use ?? ''.
TanStack Query's error is typed as Error; cast through unknown to ApiError.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/superpowers/plans/2026-04-23-web-ui-library-views.md

@@ -1631,13 +1631,13 @@ Expected: FAIL — component not found.
   import { FALLBACK_COVER } from '$lib/media/covers';
   import type { ApiError } from '$lib/api/client';
 
-  const id = $derived(page.params.id);
+  const id = $derived(page.params.id ?? '');
   const queryStore = $derived(createAlbumQuery(id));
   const query = $derived($queryStore);
   const showSkeleton = useDelayed(() => query.isPending);
 
   const notFound = $derived(
-    query.isError && (query.error as ApiError | undefined)?.code === 'not_found'
+    query.isError && (query.error as unknown as ApiError | undefined)?.code === 'not_found'
   );
 
   function onCoverError(e: Event) {