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/TransferPanel.tsx

@@ -30,7 +30,7 @@ export function TransferPanel({ tasks, isOpen, onToggle, onCancel, onRemove }: T
         <div className="flex items-center gap-2">
           <span className="text-sm font-medium">传输队列</span>
           {activeCount > 0 && (
-            <span className="px-2 py-0.5 bg-accent-blue/20 text-accent-blue text-xs rounded-full">
+            <span className="px-2 py-0.5 bg-surface-baseccent-blue/20 text-accent-blue text-xs rounded-full">
               {activeCount} 进行中
             </span>
           )}
@@ -41,7 +41,7 @@ export function TransferPanel({ tasks, isOpen, onToggle, onCancel, onRemove }: T
           )}
         </div>
         <div className="flex items-center gap-2">
-          <span className="text-xs text-gray-500">{tasks.length} 个任务</span>
+          <span className="text-xs text-txt-muted">{tasks.length} 个任务</span>
           {isOpen ? <ChevronDown className="w-4 h-4" /> : <ChevronUp className="w-4 h-4" />}
         </div>
       </button>
@@ -61,7 +61,7 @@ export function TransferPanel({ tasks, isOpen, onToggle, onCancel, onRemove }: T
       )}
 
       {isOpen && tasks.length === 0 && (
-        <div className="py-4 text-center text-gray-500 text-sm">
+        <div className="py-4 text-center text-txt-muted text-sm">
           暂无传输任务
         </div>
       )}