fix: bookmark toggle is optimistic and no longer fails silently

The detail-page bookmark toggle awaited the round-trip before updating and
had no catch — a failed create/delete threw unhandled and the user saw
nothing. It now flips optimistically (placeholder reconciled with the server
row on success), rolls back on error, and surfaces a toast, matching the
sibling like/dislike buttons.

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

frontend/e2e/detail-bookmark.spec.ts

@@ -0,0 +1,115 @@
+import { test, expect, type Page } from './fixtures';
+
+// The detail-page bookmark toggle should feel instant (optimistic) and never
+// fail silently: a rejected create/delete must roll the button back and
+// surface a toast.
+
+const mangaId = 'b1111111-1111-1111-1111-111111111111';
+
+async function mockDetail(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/me/bookmarks*', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ items: [], page: { limit: 50, offset: 0, total: 0 } })
+        })
+    );
+    await page.route(`**/api/v1/mangas/${mangaId}/chapters*`, (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ items: [], page: { limit: 50, offset: 0, total: 0 } })
+        })
+    );
+    await page.route(`**/api/v1/mangas/${mangaId}/similar`, (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify({ items: [] }) })
+    );
+    await page.route(`**/api/v1/me/read-progress/${mangaId}`, (r) =>
+        r.fulfill({ status: 404, contentType: 'application/json', body: '{"error":{"code":"x","message":"x"}}' })
+    );
+    await page.route(`**/api/v1/me/reactions/${mangaId}`, (r) =>
+        r.fulfill({ status: 404, contentType: 'application/json', body: '{"error":{"code":"x","message":"x"}}' })
+    );
+    await page.route(`**/api/v1/mangas/${mangaId}`, (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({
+                id: mangaId, title: 'Berserk', status: 'ongoing', alt_titles: [], description: null,
+                cover_image_path: null, created_at: '2026-01-01T00:00:00Z', updated_at: '2026-01-01T00:00:00Z',
+                authors: [], genres: [], tags: [], content_warnings: [], chapter_storage_bytes: 0
+            })
+        })
+    );
+}
+
+test('bookmark toggles optimistically before the request resolves', async ({ page }) => {
+    await mockDetail(page);
+
+    // Hold the create request open so we can observe the pre-response state.
+    let release: () => void = () => {};
+    const gate = new Promise<void>((resolve) => {
+        release = resolve;
+    });
+    await page.route('**/api/v1/bookmarks', async (route) => {
+        await gate;
+        await route.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({
+                id: 'bk1', user_id: 'u1', manga_id: mangaId, chapter_id: null, page: null,
+                created_at: '2026-01-01T00:00:00Z'
+            })
+        });
+    });
+
+    await page.goto(`/manga/${mangaId}`);
+    const btn = page.getByTestId('bookmark-toggle');
+    await expect(btn).toHaveAttribute('aria-pressed', 'false');
+
+    await btn.click();
+    // Optimistic: reflects bookmarked state while the POST is still pending.
+    await expect(btn).toHaveAttribute('aria-pressed', 'true');
+
+    release();
+    // Stays bookmarked once the server confirms.
+    await expect(btn).toHaveAttribute('aria-pressed', 'true');
+});
+
+test('a failed bookmark rolls back and shows an error toast', async ({ page }) => {
+    await mockDetail(page);
+    await page.route('**/api/v1/bookmarks', (route) =>
+        route.fulfill({
+            status: 500,
+            contentType: 'application/json',
+            body: JSON.stringify({ error: { code: 'internal', message: 'boom' } })
+        })
+    );
+
+    await page.goto(`/manga/${mangaId}`);
+    const btn = page.getByTestId('bookmark-toggle');
+    await btn.click();
+
+    // Rolls back to un-bookmarked, and the failure is surfaced (not silent).
+    await expect(btn).toHaveAttribute('aria-pressed', 'false');
+    await expect(page.getByTestId('toast')).toBeVisible();
+});