feat(theme): 暗黑模式 Phase 2 批次 3 — product-detail/standalone/detail-hero token 化

- product-detail-content-v3.tsx: 9 处 bg-white → bg-bg-primary
  （Hero / features / benefits / process / cases / certifications / CTA）
- standalone/[id]/client.tsx: 5 处 bg-white → bg-bg-primary
- detail-hero.tsx: 浅色 variant bg-white → bg-bg-primary（共享组件）
- detail.test.tsx: 新增常驻断言——light variant 必须用主题 token，禁止硬编码 bg-white

验证（evidence-first）:
- type-check 0 / lint 0 errors
- jest 128 suites / 1624 passed（含新增断言）
- 视觉回归 22 passed（浅色基线零 diff）
- computed-style 一次性验证 4 passed: /products/erp + /products/crm 的 dark 反色
  （html/body/header → #0A0E14、section 层次自动反色）+ light 零深色残留；脚本已删

Phase 2 剩余批次: 批次 4 admin 全量（user 决策: admin 也参与）

src/components/detail/detail-hero.tsx

@@ -32,7 +32,7 @@ export function DetailHero({
 
   const bgClass = isBrand
     ? 'bg-brand-section text-white'
-    : 'bg-white text-ink';
+    : 'bg-bg-primary text-ink';
 
   return (
     <section className={`relative min-h-[700px] flex items-center overflow-hidden ${bgClass}`}>

src/components/detail/detail.test.tsx

@@ -383,6 +383,21 @@ describe('DetailHero', () => {
     const section = container.querySelector('section');
     expect(section).toBeInTheDocument();
   });
+
+  it('should use theme-aware bg token (not hard-coded bg-white) for light variant', async () => {
+    const { DetailHero } = await import('./detail-hero');
+    const { container } = render(
+      <DetailHero
+        theme={mockHeroTheme}
+        title="ERP"
+        subtitle="Expert"
+      />
+    );
+    // 暗黑模式：浅色底色必须走 CSS 变量 token，dark 下才能随主题反色
+    const section = container.querySelector('section');
+    expect(section?.className).toContain('bg-bg-primary');
+    expect(section?.className).not.toContain('bg-white');
+  });
 });
 
 describe('ProductValueSection', () => {