fix(dark-mode): 修复视觉审查发现的2处硬编码浅色背景

- Hero右侧快速导航卡片: bg-white/80 → bg-[var(--color-bg-primary)]/80
- CTA区块: 新增--color-cta-bg变量(浅色#FAFAFA/深色#141414)
- 视觉验证通过: 8页全量截图确认无遗留问题

src/components/sections/cta-section.tsx

@@ -27,7 +27,7 @@ export function CTASection({
   const shouldReduceMotion = useReducedMotion();
 
   return (
-    <section id="cta" className="relative py-20 md:py-28 bg-[var(--color-primary)] overflow-hidden">
+    <section id="cta" className="relative py-20 md:py-28 bg-[var(--color-cta-bg)] overflow-hidden">
       <div
         className="pointer-events-none absolute inset-0"
         style={{

src/components/sections/hero-section-v2.tsx

@@ -157,7 +157,7 @@ export function HeroSectionV2() {
                 }}
               />
 
-              <div className="relative rounded-2xl bg-white/80 backdrop-blur-sm p-8 lg:p-10">
+              <div className="relative rounded-2xl bg-[var(--color-bg-primary)]/80 backdrop-blur-sm p-8 lg:p-10">
                 <div className="w-14 h-14 rounded-xl bg-[var(--color-brand-primary-bg)] flex items-center justify-center mb-6">
                   <Handshake className="w-6 h-6 text-[var(--color-brand-primary)]" strokeWidth={1.8} />
                 </div>