feat(ui/ux): 优化用户体验和可访问性

- 字体加载优化: 添加 font-display: block 策略，创建 useFontLoading hook
- 色彩对比度: 调整 text-muted 和 text-tertiary 颜色值确保 WCAG AA 合规
- 滚动进度条: 新增 ScrollProgress 组件，支持 reduced motion
- 表单自动保存: 新增 useFormAutosave hook，防止用户数据丢失
- 返回顶部按钮: 新增 BackToTop 组件，提升长页面导航体验
- 图片懒加载: 优化 OptimizedImage 组件，添加 blur placeholder 和加载动画

所有新组件均包含完整测试，1450+ 测试通过

src/components/ui/scroll-progress.test.tsx

@@ -0,0 +1,54 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import { ScrollProgress } from './scroll-progress';
+
+// Mock framer-motion
+jest.mock('framer-motion', () => ({
+  motion: {
+    div: ({ children, ...props }: React.ComponentProps<'div'>) => <div {...props}>{children}</div>,
+  },
+  useScroll: () => ({ scrollYProgress: { get: () => 0, onChange: jest.fn() } }),
+  useSpring: () => ({ get: () => 0 }),
+}));
+
+// Mock useReducedMotion
+jest.mock('@/hooks/use-reduced-motion', () => ({
+  useReducedMotion: () => false,
+}));
+
+describe('ScrollProgress', () => {
+  let scrollYValue = 0;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    scrollYValue = 0;
+    Object.defineProperty(window, 'scrollY', {
+      get: () => scrollYValue,
+      configurable: true,
+    });
+    window.addEventListener = jest.fn((event, handler) => {
+      if (event === 'scroll') {
+        (handler as EventListener)(new Event('scroll'));
+      }
+    });
+  });
+
+  it('should not render when scroll position is less than 100px', () => {
+    scrollYValue = 0;
+    const { container } = render(<ScrollProgress />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('should render progressbar with correct ARIA attributes when visible', async () => {
+    scrollYValue = 150;
+
+    render(<ScrollProgress />);
+
+    await waitFor(() => {
+      const progressbar = screen.getByRole('progressbar');
+      expect(progressbar).toBeInTheDocument();
+      expect(progressbar).toHaveAttribute('aria-label', '页面滚动进度');
+      expect(progressbar).toHaveAttribute('aria-valuemin', '0');
+      expect(progressbar).toHaveAttribute('aria-valuemax', '100');
+    });
+  });
+});