Add series fields to audiobooks and update related logic

Introduces 'series' and 'seriesPart' fields to the Audiobook model and database schema. Updates API routes, file organization, and path template utilities to support series metadata. Enhances chapter merging logic, improves notification backend testing, and expands test coverage for admin and API routes.

tests/contexts/PreferencesContext.test.tsx

@@ -0,0 +1,80 @@
+/**
+ * Component: Preferences Context Tests
+ * Documentation: documentation/frontend/components.md
+ */
+
+// @vitest-environment jsdom
+
+import React from 'react';
+import { act, fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { beforeEach, describe, expect, it } from 'vitest';
+import { PreferencesProvider, usePreferences } from '@/contexts/PreferencesContext';
+
+const TestConsumer = () => {
+  const { cardSize, setCardSize } = usePreferences();
+
+  return (
+    <div>
+      <span data-testid="size">{cardSize}</span>
+      <button type="button" onClick={() => setCardSize(12)}>
+        Set Large
+      </button>
+    </div>
+  );
+};
+
+describe('PreferencesContext', () => {
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  it('loads card size from localStorage when valid', async () => {
+    localStorage.setItem('preferences', JSON.stringify({ cardSize: 7 }));
+
+    render(
+      <PreferencesProvider>
+        <TestConsumer />
+      </PreferencesProvider>
+    );
+
+    await waitFor(() => {
+      expect(screen.getByTestId('size')).toHaveTextContent('7');
+    });
+  });
+
+  it('clamps card size updates and persists them', async () => {
+    render(
+      <PreferencesProvider>
+        <TestConsumer />
+      </PreferencesProvider>
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'Set Large' }));
+
+    await waitFor(() => {
+      expect(screen.getByTestId('size')).toHaveTextContent('9');
+    });
+
+    const stored = JSON.parse(localStorage.getItem('preferences') || '{}');
+    expect(stored.cardSize).toBe(9);
+  });
+
+  it('updates card size when a storage event is received', async () => {
+    render(
+      <PreferencesProvider>
+        <TestConsumer />
+      </PreferencesProvider>
+    );
+
+    await act(async () => {
+      window.dispatchEvent(
+        new StorageEvent('storage', {
+          key: 'preferences',
+          newValue: JSON.stringify({ cardSize: 3 }),
+        })
+      );
+    });
+
+    expect(screen.getByTestId('size')).toHaveTextContent('3');
+  });
+});