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/e2e/bookmarks-skeleton.spec.ts

@@ -0,0 +1,57 @@
+import { test, expect, type Page } from './fixtures';
+
+// The bookmarks page streams its list (the single fetch is also the auth
+// gate), so a row skeleton shows while it loads, then resolves to the list.
+
+async function mockCommon(page: Page) {
+    await page.route('**/api/v1/auth/config', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ self_register_enabled: true, private_mode: false })
+        })
+    );
+    await page.route('**/api/v1/auth/me', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ user: { id: 'u1', username: 'reader', created_at: '2026-01-01T00:00:00Z', is_admin: false } })
+        })
+    );
+    await page.route('**/api/v1/auth/me/preferences', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: '{}' })
+    );
+    await page.route('**/api/v1/files/**', (r) => r.fulfill({ status: 200, body: '' }));
+}
+
+test('shows a row skeleton while bookmarks stream, then the list', async ({ page }) => {
+    await mockCommon(page);
+
+    let release: () => void = () => {};
+    const gate = new Promise<void>((resolve) => {
+        release = resolve;
+    });
+    await page.route('**/api/v1/me/bookmarks*', async (route) => {
+        await gate;
+        await route.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({
+                items: [
+                    {
+                        id: 'bk1', user_id: 'u1', manga_id: 'm1', chapter_id: null, page: null,
+                        created_at: '2026-01-01T00:00:00Z', manga_title: 'Berserk', manga_cover_image_path: null
+                    }
+                ],
+                page: { limit: 50, offset: 0, total: 1 }
+            })
+        });
+    });
+
+    await page.goto('/bookmarks');
+    await expect(page.getByTestId('list-row-skeleton')).toBeVisible();
+
+    release();
+    await expect(page.getByText('Berserk')).toBeVisible();
+    await expect(page.getByTestId('list-row-skeleton')).toHaveCount(0);
+});