feat: skeleton loading state on the home catalog

Replaces the plain "Loading…" text with a content-shaped MangaGridSkeleton
so the catalog grid's layout is reserved while it loads and swaps in without
a shift. Keeps the data-testid="loading" wrapper (now role="status") for
accessibility and selector stability.

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

frontend/src/routes/+page.svelte

@@ -32,6 +32,7 @@
     import ContinueReadingShelf from '$lib/components/ContinueReadingShelf.svelte';
     import RecommendationShelf from '$lib/components/RecommendationShelf.svelte';
     import MangaCard from '$lib/components/MangaCard.svelte';
+    import MangaGridSkeleton from '$lib/components/MangaGridSkeleton.svelte';
     import Pager from '$lib/components/Pager.svelte';
     import SegmentedControl from '$lib/components/SegmentedControl.svelte';
     import Sheet from '$lib/components/Sheet.svelte';
@@ -659,7 +660,9 @@
 </Sheet>
 
 {#if loading}
-    <p class="status" data-testid="loading">Loading…</p>
+    <div data-testid="loading" role="status" aria-label="Loading manga">
+        <MangaGridSkeleton />
+    </div>
 {:else if error}
     <p class="error" data-testid="error" role="alert">{error}</p>
 {:else if mangas.length === 0}