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.ts

@@ -5,21 +5,27 @@ import type { PageLoad } from './$types';
 export const ssr = false;
 
 export const load: PageLoad = async () => {
-    try {
-        const page = await listMyBookmarks();
-        return { bookmarks: page.items, authenticated: true, error: null };
-    } catch (e) {
-        if (e instanceof ApiError && e.status === 401) {
-            return { bookmarks: [], authenticated: false, error: null };
-        }
-        // Anything else (502 upstream_unavailable from a backend
-        // restart, 500 internal_error) is rendered inline rather than
-        // re-thrown — SvelteKit's generic error.html is not the right
-        // UX for a transient API blip and the user is already
-        // authenticated as far as we know.
-        if (e instanceof ApiError) {
-            return { bookmarks: [], authenticated: true, error: e.message };
-        }
-        throw e;
-    }
+    // Streamed (the load itself doesn't await) so the list shows a skeleton
+    // while the single fetch — which is also the auth gate — is in flight.
+    return {
+        result: (async () => {
+            try {
+                const page = await listMyBookmarks();
+                return { bookmarks: page.items, authenticated: true, error: null as string | null };
+            } catch (e) {
+                if (e instanceof ApiError && e.status === 401) {
+                    return { bookmarks: [], authenticated: false, error: null as string | null };
+                }
+                // Anything else (502 upstream_unavailable from a backend
+                // restart, 500 internal_error) is rendered inline rather than
+                // re-thrown — SvelteKit's generic error.html is not the right
+                // UX for a transient API blip and the user is already
+                // authenticated as far as we know.
+                if (e instanceof ApiError) {
+                    return { bookmarks: [], authenticated: true, error: e.message };
+                }
+                throw e;
+            }
+        })()
+    };
 };