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/sftp/SortHeader.tsx

@@ -20,12 +20,12 @@ export function SortHeader({ onSort, sortField, sortOrder }: SortHeaderProps) {
   ]
 
   return (
-    <div className="h-9 flex items-center px-4 bg-dark-700/50 text-xs text-gray-500 border-b border-dark-700/30 select-none">
+    <div className="h-9 flex items-center px-4 bg-dark-700/50 text-xs text-txt-muted border-b border-dark-700/30 select-none">
       {fields.map(({ key, label, width, align }) => (
         <button
           key={key}
           onClick={() => onSort(key)}
-          className={`${width} flex items-center justify-${align} gap-1 hover:text-gray-200 transition-colors`}
+          className={`${width} flex items-center justify-${align} gap-1 hover:text-content-primary transition-colors`}
         >
           <span>{label}</span>
           {sortField === key && (sortOrder === 'asc' ? <ArrowUp className="w-3 h-3" /> : <ArrowDown className="w-3 h-3" />)}