feat: loading skeletons on the bookmarks and collections pages

Both loaders now stream the whole result (the single fetch is also the auth
gate) so the page shows a skeleton while it loads instead of the global nav
bar only: a ListRowSkeleton on bookmarks and a MangaGridSkeleton on the
collections grid, resolving to the sign-in / error / empty / list branches.

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

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

@@ -1,26 +1,30 @@
 <script lang="ts">
     import CollectionsGrid from '$lib/components/CollectionsGrid.svelte';
+    import MangaGridSkeleton from '$lib/components/MangaGridSkeleton.svelte';
 
     let { data } = $props();
-    const collections = $derived(data.collections);
 </script>
 
 <h1>Collections</h1>
 
-{#if !data.authenticated}
-    <p class="status">
-        <a href="/login">Sign in</a> to see and manage your collections.
-    </p>
-{:else if data.error}
-    <p class="error" role="alert">{data.error}</p>
-{:else if collections.length === 0}
-    <p class="status" data-testid="collections-empty">
-        You don't have any collections yet. Open any manga and use
-        <strong>Add to collection</strong> to start one.
-    </p>
-{:else}
-    <CollectionsGrid {collections} />
-{/if}
+{#await data.result}
+    <MangaGridSkeleton count={6} />
+{:then r}
+    {#if !r.authenticated}
+        <p class="status">
+            <a href="/login">Sign in</a> to see and manage your collections.
+        </p>
+    {:else if r.error}
+        <p class="error" role="alert">{r.error}</p>
+    {:else if r.collections.length === 0}
+        <p class="status" data-testid="collections-empty">
+            You don't have any collections yet. Open any manga and use
+            <strong>Add to collection</strong> to start one.
+        </p>
+    {:else}
+        <CollectionsGrid collections={r.collections} />
+    {/if}
+{/await}
 
 <style>
     .status {