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/Breadcrumb.tsx

@@ -20,11 +20,11 @@ export function Breadcrumb({ path, onNavigate, onHome }: BreadcrumbProps) {
         className="p-1.5 hover:bg-dark-700 rounded transition-colors flex-shrink-0"
         title="主目录"
       >
-        <Home className="w-4 h-4 text-gray-400" />
+        <Home className="w-4 h-4 text-txt-muted" />
       </button>
       <button
         onClick={() => onNavigate('/')}
-        className="px-1.5 py-0.5 hover:bg-dark-700 rounded text-gray-400 hover:text-dark-200 transition-colors"
+        className="px-1.5 py-0.5 hover:bg-dark-700 rounded text-txt-muted hover:text-dark-200 transition-colors"
       >
         /
       </button>
@@ -33,7 +33,7 @@ export function Breadcrumb({ path, onNavigate, onHome }: BreadcrumbProps) {
           <ChevronRight className="w-3 h-3 text-dark-600 mx-0.5 flex-shrink-0" />
           <button
             onClick={() => onNavigate('/' + parts.slice(0, index + 1).join('/'))}
-            className="px-1.5 py-0.5 hover:bg-dark-700 rounded text-gray-200 hover:text-white transition-colors truncate max-w-[120px]"
+            className="px-1.5 py-0.5 hover:bg-dark-700 rounded text-content-primary hover:text-content-primary transition-colors truncate max-w-[120px]"
           >
             {part}
           </button>