实现跨边界拖拽功能：支持从浏览器拖拽文件到系统

主要改进：
- 使用 File System Access API 实现真正的跨应用边界文件传输
- 支持拖拽到窗口外自动触发系统保存对话框
- 智能路径记忆功能，记住用户上次选择的保存位置
- 多文件自动打包为 ZIP 格式
- 现代浏览器优先使用新 API，旧浏览器降级到传统下载
- 完整的视觉反馈和进度显示

技术实现：
- 新增 useDragToSystemDesktop hook 处理系统级拖拽
- 扩展 Electron 主进程支持拖拽临时文件管理
- 集成 JSZip 库支持多文件打包
- 使用 IndexedDB 存储用户偏好的保存路径
- 优化文件管理器拖拽事件处理链

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

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

src/ui/components/DragIndicator.tsx

@@ -0,0 +1,140 @@
+import React from 'react';
+import { cn } from '@/lib/utils';
+import {
+  Download,
+  FileDown,
+  FolderDown,
+  Loader2,
+  CheckCircle,
+  AlertCircle
+} from 'lucide-react';
+
+interface DragIndicatorProps {
+  isVisible: boolean;
+  isDragging: boolean;
+  isDownloading: boolean;
+  progress: number;
+  fileName?: string;
+  fileCount?: number;
+  error?: string | null;
+  className?: string;
+}
+
+export function DragIndicator({
+  isVisible,
+  isDragging,
+  isDownloading,
+  progress,
+  fileName,
+  fileCount = 1,
+  error,
+  className
+}: DragIndicatorProps) {
+  if (!isVisible) return null;
+
+  const getIcon = () => {
+    if (error) {
+      return <AlertCircle className="w-6 h-6 text-red-500" />;
+    }
+
+    if (isDragging) {
+      return <CheckCircle className="w-6 h-6 text-green-500" />;
+    }
+
+    if (isDownloading) {
+      return <Loader2 className="w-6 h-6 text-blue-500 animate-spin" />;
+    }
+
+    if (fileCount > 1) {
+      return <FolderDown className="w-6 h-6 text-blue-500" />;
+    }
+
+    return <FileDown className="w-6 h-6 text-blue-500" />;
+  };
+
+  const getStatusText = () => {
+    if (error) {
+      return `错误: ${error}`;
+    }
+
+    if (isDragging) {
+      return `正在拖拽${fileName ? ` ${fileName}` : ''}到桌面...`;
+    }
+
+    if (isDownloading) {
+      return `正在准备拖拽${fileName ? ` ${fileName}` : ''}...`;
+    }
+
+    return `准备拖拽${fileCount > 1 ? ` ${fileCount} 个文件` : fileName ? ` ${fileName}` : ''}`;
+  };
+
+  return (
+    <div
+      className={cn(
+        "fixed top-4 right-4 z-50 min-w-[300px] max-w-[400px]",
+        "bg-dark-bg border border-dark-border rounded-lg shadow-lg",
+        "p-4 transition-all duration-300 ease-in-out",
+        isVisible ? "opacity-100 translate-x-0" : "opacity-0 translate-x-full",
+        className
+      )}
+    >
+      <div className="flex items-start gap-3">
+        {/* 图标 */}
+        <div className="flex-shrink-0 mt-0.5">
+          {getIcon()}
+        </div>
+
+        {/* 内容 */}
+        <div className="flex-1 min-w-0">
+          {/* 标题 */}
+          <div className="text-sm font-medium text-foreground mb-2">
+            {fileCount > 1 ? '批量拖拽到桌面' : '拖拽到桌面'}
+          </div>
+
+          {/* 状态文字 */}
+          <div className={cn(
+            "text-xs mb-3",
+            error ? "text-red-500" :
+            isDragging ? "text-green-500" :
+            "text-muted-foreground"
+          )}>
+            {getStatusText()}
+          </div>
+
+          {/* 进度条 */}
+          {(isDownloading || isDragging) && !error && (
+            <div className="w-full bg-dark-border rounded-full h-2 mb-2">
+              <div
+                className={cn(
+                  "h-2 rounded-full transition-all duration-300",
+                  isDragging ? "bg-green-500" : "bg-blue-500"
+                )}
+                style={{ width: `${Math.max(5, progress)}%` }}
+              />
+            </div>
+          )}
+
+          {/* 进度百分比 */}
+          {(isDownloading || isDragging) && !error && (
+            <div className="text-xs text-muted-foreground">
+              {progress.toFixed(0)}%
+            </div>
+          )}
+
+          {/* 拖拽提示 */}
+          {isDragging && !error && (
+            <div className="text-xs text-green-500 mt-2 flex items-center gap-1">
+              <Download className="w-3 h-3" />
+              现在可以拖拽到桌面任意位置
+            </div>
+          )}
+        </div>
+      </div>
+
+      {/* 动画效果的背景 */}
+      {isDragging && !error && (
+        <div className="absolute inset-0 rounded-lg bg-green-500/5 animate-pulse" />
+      )}
+    </div>
+  );
+}