fix: paginate the bookmarks and collections lists with Load more

Both lists fetched a single capped page (100 / 200) with no pager, so entries
past the cap were silently unreachable. Add a reusable LoadMore component that
seeds from the streamed first page (preserving the auth gate + inline error
handling) and fetches further offset-based pages on demand, and wire it into
the bookmarks and collections pages.

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

frontend/src/routes/collections/+page.svelte

@@ -1,8 +1,15 @@
 <script lang="ts">
     import CollectionsGrid from '$lib/components/CollectionsGrid.svelte';
     import MangaGridSkeleton from '$lib/components/MangaGridSkeleton.svelte';
+    import LoadMore from '$lib/components/LoadMore.svelte';
+    import { listMyCollections, type CollectionSummary } from '$lib/api/collections';
 
     let { data } = $props();
+
+    async function fetchMore(offset: number) {
+        const p = await listMyCollections({ limit: data.pageSize, offset });
+        return { items: p.items, total: p.page.total };
+    }
 </script>
 
 <h1>Collections</h1>
@@ -22,7 +29,11 @@
             <strong>Add to collection</strong> to start one.
         </p>
     {:else}
-        <CollectionsGrid collections={r.collections} />
+        <LoadMore initial={r.collections as CollectionSummary[]} total={r.total} {fetchMore}>
+            {#snippet children(items: CollectionSummary[])}
+                <CollectionsGrid collections={items} />
+            {/snippet}
+        </LoadMore>
     {/if}
 {/await}