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/components/ui/alert-dialog.tsx

@@ -21,7 +21,7 @@ const AlertDialogOverlay = React.forwardRef<
     ref={ref}
     data-slot="alert-dialog-overlay"
     className={cn(
-      'fixed inset-0 z-50 bg-ink/60 backdrop-blur-sm',
+      'fixed inset-0 z-50 bg-overlay/60 backdrop-blur-sm',
       'data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0',
       className
     )}

src/components/ui/button.test.tsx

@@ -28,8 +28,8 @@ describe('Button Component', () => {
     it('should apply secondary variant styles', () => {
       render(<Button variant="secondary">Secondary</Button>);
       const button = screen.getByRole('button');
-      // secondary variant 使用 bg-ink
-      expect(button).toHaveClass('bg-ink');
+      // secondary variant 使用 bg-dark-bg（深色区块 token，不随暗黑模式反色）
+      expect(button).toHaveClass('bg-dark-bg');
     });
 
     it('should apply outline variant styles', () => {

src/components/ui/button.tsx

@@ -13,9 +13,9 @@ const buttonVariants = cva(
         primary:
           'bg-brand text-white shadow-sm hover:bg-brand-hover hover:shadow-brand-hover hover:-translate-y-px active:translate-y-0 active:scale-[0.97] active:shadow-sm transition-all duration-fast ease-standard',
         secondary:
-          'bg-ink text-white shadow-sm hover:bg-ink-light hover:shadow-md hover:-translate-y-px active:translate-y-0 active:scale-[0.97] active:shadow-sm transition-all duration-fast ease-standard',
+          'bg-dark-bg text-white shadow-sm hover:bg-dark-bg-secondary hover:shadow-md hover:-translate-y-px active:translate-y-0 active:scale-[0.97] active:shadow-sm transition-all duration-fast ease-standard',
         outline:
-          'border border-border-accent bg-transparent text-text-primary hover:bg-ink hover:text-white hover:border-ink hover:-translate-y-px hover:shadow-md active:translate-y-0 active:scale-[0.97] active:shadow-sm transition-all duration-fast ease-standard',
+          'border border-border-accent bg-transparent text-text-primary hover:bg-dark-bg hover:text-white hover:border-dark-bg hover:-translate-y-px hover:shadow-md active:translate-y-0 active:scale-[0.97] active:shadow-sm transition-all duration-fast ease-standard',
         ghost:
           'text-text-secondary hover:bg-bg-secondary hover:text-text-primary active:scale-[0.97] transition-all duration-fast ease-standard',
         link:

src/components/ui/dialog.tsx

@@ -22,7 +22,7 @@ const DialogOverlay = React.forwardRef<
     ref={ref}
     data-slot="dialog-overlay"
     className={cn(
-      'fixed inset-0 z-50 bg-ink/60 backdrop-blur-sm',
+      'fixed inset-0 z-50 bg-overlay/60 backdrop-blur-sm',
       'data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0',
       className
     )}

src/components/ui/metric-card.tsx

@@ -66,7 +66,7 @@ export function MetricCard({
       className={cn(
         'group relative overflow-hidden rounded-sm border p-6 sm:p-7 transition-all duration-300 ease-out',
         isDark
-          ? 'bg-ink/50 border-white/[0.08] hover:border-white/20 hover:bg-ink'
+          ? 'bg-overlay/50 border-white/[0.08] hover:border-white/20 hover:bg-overlay'
           : 'border-border-primary bg-bg-primary hover:bg-bg-secondary',
         className
       )}

src/components/ui/milestone-timeline.tsx

@@ -64,7 +64,7 @@ export function MilestoneTimeline({
             >
               <div className={cn(
                 'absolute left-2 top-1 w-5 h-5 rounded-full border-2 flex items-center justify-center',
-                isDark ? 'bg-ink border-brand' : 'bg-bg-primary border-brand'
+                isDark ? 'bg-dark-bg border-brand' : 'bg-bg-primary border-brand'
               )}>
                 <div className="w-2 h-2 rounded-full bg-brand" />
               </div>

src/components/ui/stats-showcase.tsx

@@ -73,7 +73,7 @@ export function StatsShowcase({
           key={i}
           className={cn(
             'relative p-8 sm:p-10 md:p-12 text-center transition-all duration-500 group',
-            isDark ? 'bg-ink hover:bg-ink-light' : 'bg-bg-primary hover:bg-bg-hover'
+            isDark ? 'bg-dark-bg hover:bg-dark-bg-secondary' : 'bg-bg-primary hover:bg-bg-hover'
           )}
         >
           <div className="relative z-10">