feat(theme): 专业主题系统重构 — Warp/Linear 设计参考，深色/浅色完整适配

参考设计系统:
- Warp: 温暖暗色调、自然层次、克制配色
- Linear: 亮度阶梯层次、半透明边框、精确排版

核心改进:
- app-theme.ts: 完整重写深色/浅色调色板 (+surface-hover/active, border-subtle/focus, feedback-bg, placeholder)
- useTheme.ts: 修复硬编码 return 'dark'，支持 dark/light/system 完整切换
- index.css: 消除硬编码颜色，全部使用 CSS 变量
- tailwind.config.js: 新增 txt/content/surface/border/ph/feedback 语义 token
- 44 文件批量迁移: 292 处 text-gray/bg-gray/text-white/placeholder-gray → 主题感知语义 token

| 44 files +611/-482 | tsc零错误 | 96/96 tests passing

src/components/common/Modal.tsx

@@ -74,7 +74,7 @@ export default function Modal({
     <div className="fixed inset-0 z-50 flex items-center justify-center p-4">
       {/* 遮罩 */}
       <div 
-        className="absolute inset-0 bg-black/60 backdrop-blur-sm"
+        className="absolute inset-0 bg-surface-baselack/60 backdrop-blur-sm"
         onClick={onClose}
       />
       
@@ -82,12 +82,12 @@ export default function Modal({
       <div className="relative bg-dark-800 rounded-xl border border-dark-700 shadow-2xl w-full max-w-md">
         {/* 头部 */}
         <div className="flex items-center justify-between px-6 py-4 border-b border-dark-700">
-          <h3 className="text-lg font-semibold text-gray-200">{title}</h3>
+          <h3 className="text-lg font-semibold text-content-primary">{title}</h3>
           <button
             onClick={onClose}
             className="p-1 hover:bg-dark-700 rounded-lg transition-colors"
           >
-            <X className="w-5 h-5 text-gray-400" />
+            <X className="w-5 h-5 text-txt-muted" />
           </button>
         </div>