feat(theme): 暗黑模式 Phase 1 基础设施 + 首页试点

为 html[data-theme='dark'] 添加完整 CSS 变量覆盖块，建立暗黑模式基石。
--color-ink 收窄为'文字/描边跟随主题'语义，深色 UI 元素（卡片/按钮/遮罩/
代码块/页脚）迁到 --color-dark-bg / --color-overlay 独立 token，避免 ink
反色为浅色文字时连带把深色区块变浅（双语义冲突）。

【基础设施】
- globals.css：
  * html/body 顶部硬编码 #FFFFFF !important 改 var() 化（保留 !important）
  * :root 新增 --color-overlay / --color-overlay-rgb（遮罩/深色叠加层，
    永不反色）
  * 新增 html[data-theme='dark'] 覆盖块：墨色反色为浅色文字，bg/text/
    border 层级反色，link 反色；保留 --color-dark-* / --color-brand-section
  * pre 背景 var(--color-ink) → var(--color-dark-bg)
- tailwind.config.js：注册 overlay 色（rgb() 形式，支持 /50 /60 修饰符）
- layout.tsx：head 内联防 FOUC 同步脚本，读 localStorage['novalon-theme']
  设置 data-theme（当前无切换 UI，仅作未来挂载点）

【首页试点 + 布局组件】（浅色值完全等价 → 浅色零视觉差异）
- home-content-v15.tsx：7 处 bg-white → bg-bg-primary（narrative/insights/
  cases sections 与卡片），2 处 bg-ink → bg-dark-bg（案例卡 bg-dark-bg 不
  随反色，保留深色画布）
- header.tsx / mega-dropdown.tsx：bg-white → bg-bg-primary（导航栏/移动
  端菜单/下拉面板）
- footer.tsx：bg-[#0A0E14] → bg-dark-bg（语义统一，视觉零差异）

【全局 UI 组件】（深色区块语义统一 + 遮罩固定深色）
- button.tsx：secondary 'bg-ink hover:bg-ink-light' → bg-dark-bg / hover:bg-dark-bg-secondary；
  outline 'hover:bg-ink hover:border-ink' → hover:bg-dark-bg / border-dark-bg
- stats-showcase / milestone-timeline：isDark 分支 bg-ink → bg-dark-bg
- metric-card：isDark 分支 bg-ink/50 → bg-overlay/50，hover:bg-ink → hover:bg-overlay
- dialog / alert-dialog：遮罩 bg-ink/60 → bg-overlay/60（永不反色）
- button.test.tsx：断言 bg-ink → bg-dark-bg，注释同步

【验证】
- type-check 0 errors
- lint 0 errors（127 warnings 既有，非本次引入）
- jest 128 suites / 1623 passed（button 断言同步通过）
- 视觉回归先非 update：21 passed（浅色安全门零 diff）+ 1 failed
  （仅 theme-dark-main，dark 快照从浅色名义变真正深色 = 语义修复预期）
- update-snapshots：22 passed，仅 theme-dark-main 基线重写
- computed-style 权威验证（一次性脚本，已删除）：
  dark 7 项断言全过（html/body/hero/导航栏/案例卡→深色 #0A0E14；
  body 文字→浅色 #F8FAFC；footer/overlay→保持深色）；light 4 项零回归

【未在 Phase 1 范围】（已识别，留待 Phase 2/3 决策）
- service-detail-content-v4.tsx 等营销页 bg-white：Phase 2 批量 token 化
- admin 页面 bg-white + RichTextEditor：admin 边界决策 + Phase 3
- 品牌红 #C41E3A 在深色底对比度复检：Phase 3

src/app/(marketing)/home-content-v15.tsx

@@ -333,7 +333,7 @@ function TrustSection({ pageCopy }: { pageCopy?: HomePageCopy | null }) {
             </p>
             <div className="grid sm:grid-cols-3 gap-4 mb-8">
               {earlyStatus.map((s) => (
-                <div key={s.label} className="p-4 bg-white border border-brand/10">
+                <div key={s.label} className="p-4 bg-bg-primary border border-brand/10">
                   <div className="text-sm font-semibold text-brand mb-1">{s.label}</div>
                   <div className="text-xs text-text-secondary leading-relaxed">{s.desc}</div>
                 </div>
@@ -389,7 +389,7 @@ function NarrativeSection({ pageCopy }: { pageCopy?: HomePageCopy | null }) {
   }>;
 
   return (
-    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-white" data-testid="narrative-section">
+    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-bg-primary" data-testid="narrative-section">
       <div className="max-w-container mx-auto px-4 sm:px-6 lg:px-10 relative z-10">
         <ScrollReveal className="mb-16 sm:mb-20">
           <h2 className="text-2xl sm:text-3xl md:text-4xl lg:text-5xl font-black text-ink tracking-tight leading-[1.05]">
@@ -400,7 +400,7 @@ function NarrativeSection({ pageCopy }: { pageCopy?: HomePageCopy | null }) {
         <div className="grid md:grid-cols-3 gap-px bg-border-primary">
           {NARRATIVE_ACTS_CMS.map((act) => (
             <StaggerReveal key={act.number} staggerDelay={0.08}>
-              <div className="group relative h-full bg-white p-8 sm:p-10 md:p-12 transition-colors duration-300 hover:bg-bg-secondary">
+              <div className="group relative h-full bg-bg-primary p-8 sm:p-10 md:p-12 transition-colors duration-300 hover:bg-bg-secondary">
                 <div className="flex items-start justify-between mb-8">
                   <span
                     aria-hidden="true"
@@ -447,7 +447,7 @@ function InsightsSection({ pageCopy }: { pageCopy?: HomePageCopy | null }) {
   }>;
 
   return (
-    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-white" data-testid="insights-section">
+    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-bg-primary" data-testid="insights-section">
       <div className="max-w-container mx-auto px-4 sm:px-6 lg:px-10 relative z-10">
         <div className="flex flex-col md:flex-row md:items-end md:justify-between gap-8 mb-16 sm:mb-20">
           <ScrollReveal className="md:max-w-3xl">
@@ -480,7 +480,7 @@ function InsightsSection({ pageCopy }: { pageCopy?: HomePageCopy | null }) {
             <StaggerReveal key={i} staggerDelay={0.07}>
               <a
                 href={insight.href}
-                className="group flex flex-col h-full bg-white p-8 sm:p-10 md:p-12 transition-colors duration-300 hover:bg-bg-secondary"
+                className="group flex flex-col h-full bg-bg-primary p-8 sm:p-10 md:p-12 transition-colors duration-300 hover:bg-bg-secondary"
               >
                 <div className="flex items-center justify-between mb-8">
                   <span className="text-[13px] tracking-[0.12em] font-semibold text-brand">
@@ -645,7 +645,7 @@ function CasesSection({ cases, pageCopy }: { cases?: Record<string, any>[]; page
 
   if (CASE_ITEMS.length === 0) {
     return (
-      <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-white">
+      <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-bg-primary">
         <div className="max-w-container mx-auto px-4 sm:px-6 lg:px-10 relative z-10">
           <div className="grid lg:grid-cols-2 gap-12 items-center">
             <ScrollReveal>
@@ -670,7 +670,7 @@ function CasesSection({ cases, pageCopy }: { cases?: Record<string, any>[]; page
               </StaticLink>
             </ScrollReveal>
             <ScrollReveal delay={0.15}>
-              <div className="relative overflow-hidden rounded-[2rem] bg-ink p-10 sm:p-12 h-full flex flex-col shadow-card">
+              <div className="relative overflow-hidden rounded-[2rem] bg-dark-bg p-10 sm:p-12 h-full flex flex-col shadow-card">
                 {/* 背景纹理：极淡白色点阵（对标埃森哲深色 spotlight 画布） */}
                 <div
                   className="absolute inset-0 pointer-events-none opacity-[0.05]"
@@ -722,7 +722,7 @@ function CasesSection({ cases, pageCopy }: { cases?: Record<string, any>[]; page
   const spotlightTestimonial = spotlightCase?.testimonial as Record<string, any> | undefined;
 
   return (
-    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-white">
+    <section className="relative py-20 sm:py-28 md:py-36 overflow-hidden bg-bg-primary">
       <div className="max-w-container mx-auto px-4 sm:px-6 lg:px-10 relative z-10">
         <div className="flex flex-col md:flex-row md:items-end md:justify-between gap-8 mb-16 sm:mb-20">
           <ScrollReveal className="md:max-w-3xl">
@@ -746,7 +746,7 @@ function CasesSection({ cases, pageCopy }: { cases?: Record<string, any>[]; page
           <ScrollReveal className="mb-6">
             <a
               href={spotlightCase.href || `/cases/${spotlightCase.slug || ''}`}
-              className="group relative block overflow-hidden rounded-[2rem] bg-ink p-10 sm:p-14 h-full flex flex-col shadow-card"
+              className="group relative block overflow-hidden rounded-[2rem] bg-dark-bg p-10 sm:p-14 h-full flex flex-col shadow-card"
             >
               {/* 背景纹理：极淡白色点阵 */}
               <div