feat: grid skeletons on the author and collection detail pages

Both loaders now stream their manga grid instead of blocking on it: the cheap
header renders immediately (and still handles 404/401 in the loader) while a
MangaGridSkeleton stands in until the grid resolves, then swaps in without a
layout shift. Collection detail seeds its optimistic-removal state from the
stream via a guarded effect.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/collections/[id]/+page.ts

@@ -10,18 +10,11 @@ import type { PageLoad } from './$types';
 export const ssr = false;
 
 export const load: PageLoad = async ({ params, url }) => {
+    let collection;
     try {
-        const [collection, mangas, pages] = await Promise.all([
-            getCollection(params.id),
-            listCollectionMangas(params.id, { limit: 200 }),
-            listCollectionPages(params.id, { limit: 200 })
-        ]);
-        return {
-            collection,
-            mangas: mangas.items,
-            total: mangas.page.total,
-            pages: pages.items
-        };
+        // Awaited so the header renders immediately and 401/404 are handled by
+        // the framework (redirect / not-found) rather than the happy path.
+        collection = await getCollection(params.id);
     } catch (e) {
         if (e instanceof ApiError) {
             if (e.status === 401) {
@@ -34,4 +27,13 @@ export const load: PageLoad = async ({ params, url }) => {
         }
         throw e;
     }
+    return {
+        collection,
+        // Streamed so the manga/page grids show a skeleton while they load
+        // instead of the whole page blocking on them.
+        content: Promise.all([
+            listCollectionMangas(params.id, { limit: 200 }),
+            listCollectionPages(params.id, { limit: 200 })
+        ]).then(([mangas, pages]) => ({ mangas: mangas.items, pages: pages.items }))
+    };
 };