优化文件管理器视觉设计和安全性

核心改进：
- 统一图标色调：所有文件类型图标改为黑白配色，消除彩色差异
- 恢复原始主题：修复shadcn样式导致的过暗背景问题
- 增强大文件安全：后端10MB文件大小限制，防止内存溢出
- 优化警告样式：Large File Warning使用shadcn设计规范

技术细节：
- getFileIcon()全部使用text-muted-foreground统一色调
- 恢复bg-dark-bg/border-dark-border原始主题色彩
- readFile API增加stat文件大小检查和错误处理
- FileViewer警告组件使用destructive色彩体系

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/ui/Desktop/Apps/File Manager/components/FileViewer.tsx

@@ -579,17 +579,17 @@ export function FileViewer({
         {/* 大文件警告对话框 */}
         {showLargeFileWarning && (
           <div className="h-full flex items-center justify-center bg-background">
-            <div className="bg-card border border-orange-200 rounded-lg p-6 max-w-md mx-4 shadow-lg">
+            <div className="bg-card border border-destructive/30 rounded-lg p-6 max-w-md mx-4 shadow-lg">
               <div className="flex items-start gap-3 mb-4">
-                <AlertCircle className="w-6 h-6 text-orange-500 flex-shrink-0 mt-0.5" />
+                <AlertCircle className="w-6 h-6 text-destructive flex-shrink-0 mt-0.5" />
                 <div>
                   <h3 className="font-medium text-foreground mb-2">Large File Warning</h3>
                   <p className="text-sm text-muted-foreground mb-3">
                     This file is {formatFileSize(file.size)} in size, which may cause performance issues when opened as text.
                   </p>
                   {isTooLarge ? (
-                    <div className="bg-red-50 border border-red-200 rounded p-3 mb-4">
-                      <p className="text-sm text-red-700 font-medium">
+                    <div className="bg-destructive/10 border border-destructive/30 rounded p-3 mb-4">
+                      <p className="text-sm text-destructive font-medium">
                         File is too large (&gt; 10MB) and cannot be opened as text for security reasons.
                       </p>
                     </div>