fix(theme): 品牌色令牌语义拆分与暗黑模式全站治理

- 拆分品牌色「文字/底色」双通道：新增 --color-brand-ink(-rgb)（深色 #F87171）
  与 tailwind brand.ink；text-brand → text-brand-ink 迁移 247 处；
  bg-brand/border-brand 保持 --color-brand-rgb 不翻转（红底白字 5.58:1 不能动）
- dark 块补齐 --color-brand-bg: #2A1418（修复深色白字压浅粉底 1.04:1）
- hero 徽章新增 --badge-accent-text 桥接：accentColor 直用作深底文字不可读
  （#1e3a5f 深底 1.65:1），浅色取原色 / 深色 color-mix 提亮
- 硬编码颜色 token 化：bg-white/text-gray-*/border-gray-* → bg-bg-*/text-text-*/border-border-*
- 可访问性：触控目标 <24px 归零（footer 链接列 [&_a]:min-h-6 收口 377 处等）、
  news 搜索框补 aria-label、标题跳级 h2→h4 修正为 h3、装饰 blur 光斑容器补
  overflow-hidden 消除伪文本裁剪
- 验证：tsc 0 / eslint 0 error / jest 0 失败用例 / dogfood 33 路由 × 双主题 P1-P3 全零

src/app/(marketing)/cases/cases-content-v3.tsx

@@ -32,7 +32,7 @@ function CaseCard({ caseItem, index }: { caseItem: CaseStudy; index: number }) {
 
       <div className="p-6 sm:p-7 lg:p-8 flex flex-col flex-1 relative z-10">
         <div className="flex items-center gap-3 mb-5">
-          <span className="inline-block px-3 py-1 text-[11px] font-bold text-brand border border-brand/20 bg-brand/[0.05]">
+          <span className="inline-block px-3 py-1 text-[11px] font-bold text-brand-ink border border-brand/20 bg-brand/[0.05]">
             {caseItem.industry}
           </span>
           <span className="text-[10px] tracking-[0.25em] uppercase text-text-muted font-semibold">
@@ -40,15 +40,15 @@ function CaseCard({ caseItem, index }: { caseItem: CaseStudy; index: number }) {
           </span>
         </div>
 
-        <h3 className="text-lg sm:text-xl font-bold text-ink mb-4 leading-tight group-hover:text-brand transition-colors duration-300">
+        <h3 className="text-lg sm:text-xl font-bold text-ink mb-4 leading-tight group-hover:text-brand-ink transition-colors duration-300">
           {caseItem.title}
         </h3>
 
         <div className="space-y-4 mb-6 flex-1">
           <div>
             <div className="flex items-center gap-2 mb-2">
-              <Target className="w-3.5 h-3.5 text-brand" />
-              <span className="text-[11px] tracking-wide uppercase font-bold text-brand">挑战</span>
+              <Target className="w-3.5 h-3.5 text-brand-ink" />
+              <span className="text-[11px] tracking-wide uppercase font-bold text-brand-ink">挑战</span>
             </div>
             <p className="text-sm text-text-secondary leading-relaxed line-clamp-2 pl-5.5">
               {caseItem.challenge}
@@ -69,7 +69,7 @@ function CaseCard({ caseItem, index }: { caseItem: CaseStudy; index: number }) {
           {caseItem.metrics.slice(0, 2).map((m, i) => (
             <div key={i}>
               <div className={`text-2xl sm:text-3xl font-black mb-1 tabular-nums tracking-tight ${
-                i === 0 ? 'text-brand' : 'text-ink'
+                i === 0 ? 'text-brand-ink' : 'text-ink'
               }`}>
                 {m.value}
               </div>
@@ -83,7 +83,7 @@ function CaseCard({ caseItem, index }: { caseItem: CaseStudy; index: number }) {
             <Building2 className="w-4 h-4 text-text-muted" />
             <span className="text-xs text-text-muted">{caseItem.client}</span>
           </div>
-          <div className="flex items-center gap-1.5 text-brand text-sm font-semibold">
+          <div className="flex items-center gap-1.5 text-brand-ink text-sm font-semibold">
             查看详情
             <ArrowUpRight className="w-4 h-4 group-hover:translate-x-0.5 group-hover:-translate-y-0.5 transition-transform duration-300" />
           </div>
@@ -143,7 +143,7 @@ export default function CasesContentV3({
               <div className="block">{heroTitle1}</div>
               <div className="block mt-4 sm:mt-6">
                 <span className="relative inline-block">
-                  <span className="relative text-brand font-black">
+                  <span className="relative text-brand-ink font-black">
                     {heroTitle2}
                     <motion.span
                       className="absolute -bottom-2 left-0 w-full h-[3px] bg-brand"
@@ -177,7 +177,7 @@ export default function CasesContentV3({
               transition={{ duration: 0.3, delay: 0.7, ease: EASE_OUT }}
               className="flex flex-wrap gap-3"
             >
-              <span className="px-4 py-2 text-xs font-bold text-brand border border-brand/20 bg-brand/[0.05]">
+              <span className="px-4 py-2 text-xs font-bold text-brand-ink border border-brand/20 bg-brand/[0.05]">
                 首批客户共创中
               </span>
               <span className="px-4 py-2 text-xs font-medium text-text-secondary border border-border-primary">
@@ -211,7 +211,7 @@ export default function CasesContentV3({
                 className="text-center lg:text-left"
               >
                 <div className={`text-3xl sm:text-4xl md:text-5xl font-black tracking-tight mb-2 ${
-                  i === 0 ? 'text-brand' : 'text-ink'
+                  i === 0 ? 'text-brand-ink' : 'text-ink'
                 }`}>
                   {metric.value}
                 </div>