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/bookmarks/+page.svelte

@@ -1,27 +1,29 @@
 <script lang="ts">
     import BookmarkList from '$lib/components/BookmarkList.svelte';
+    import ListRowSkeleton from '$lib/components/ListRowSkeleton.svelte';
 
     let { data } = $props();
-    const authenticated = $derived(data.authenticated);
-    const bookmarks = $derived(data.bookmarks);
-    const error = $derived(data.error);
 </script>
 
 <h1>Bookmarks</h1>
 
-{#if error}
-    <p class="error" role="alert" data-testid="bookmarks-error">
-        Couldn't load bookmarks: {error}
-    </p>
-{:else if !authenticated}
-    <p class="hint" data-testid="bookmarks-signin">
-        <a href="/login">Sign in</a> to see your bookmarks.
-    </p>
-{:else if bookmarks.length === 0}
-    <p class="hint" data-testid="bookmarks-empty">No bookmarks yet.</p>
-{:else}
-    <BookmarkList {bookmarks} />
-{/if}
+{#await data.result}
+    <ListRowSkeleton />
+{:then r}
+    {#if r.error}
+        <p class="error" role="alert" data-testid="bookmarks-error">
+            Couldn't load bookmarks: {r.error}
+        </p>
+    {:else if !r.authenticated}
+        <p class="hint" data-testid="bookmarks-signin">
+            <a href="/login">Sign in</a> to see your bookmarks.
+        </p>
+    {:else if r.bookmarks.length === 0}
+        <p class="hint" data-testid="bookmarks-empty">No bookmarks yet.</p>
+    {:else}
+        <BookmarkList bookmarks={r.bookmarks} />
+    {/if}
+{/await}
 
 <style>
     .error {