fix(a11y): 无障碍与 lint 修复（对比度/alt 属性/触控目标收窄）+ Geist 本地字体系统

src/components/sections/case-card.test.tsx

@@ -83,16 +83,18 @@ describe('CaseCard', () => {
   it('should render background image when provided', async () => {
     const { CaseCard } = await import('./case-card');
     const { container } = render(<CaseCard {...mockCase} />);
-    const img = container.querySelector('img[alt=""]');
-    expect(img).toBeInTheDocument();
-    expect(img).toHaveAttribute('src', '/images/cases/manufacturing.jpg');
+    // 组件用 CSS 背景图而非 <img>，使 404 图片静默降级为纯色底
+    const bg = container.querySelector('[style*="background-image"]');
+    expect(bg).toBeInTheDocument();
+    expect(bg).toHaveAttribute('aria-hidden', 'true');
+    expect(bg).toHaveStyle({ backgroundImage: 'url(/images/cases/manufacturing.jpg)' });
   });
 
   it('should not render background image when not provided', async () => {
     const { CaseCard } = await import('./case-card');
     const { container } = render(<CaseCard {...mockCase} image={undefined} />);
-    const img = container.querySelector('img[alt=""]');
-    expect(img).not.toBeInTheDocument();
+    const bg = container.querySelector('[style*="background-image"]');
+    expect(bg).not.toBeInTheDocument();
   });
 
   it('should use context as fallback when challenge is empty', async () => {

src/components/sections/insight-card.test.tsx

@@ -75,9 +75,11 @@ describe('InsightCard', () => {
     const { container } = render(
       <InsightCard {...mockInsight} featured image="/images/insights/trends.jpg" />
     );
-    const img = container.querySelector('img');
-    expect(img).toBeInTheDocument();
-    expect(img).toHaveAttribute('src', '/images/insights/trends.jpg');
+    // 组件用 CSS 背景图而非 <img>，使 404 图片静默降级为纯色底
+    const bg = container.querySelector('[style*="background-image"]');
+    expect(bg).toBeInTheDocument();
+    expect(bg).toHaveAttribute('aria-hidden', 'true');
+    expect(bg).toHaveStyle({ backgroundImage: 'url(/images/insights/trends.jpg)' });
   });
 
   it('should render link element when provided', async () => {