fix: 限制账户导出框高度，防止按钮被遮挡

- 设置导出对话框最大高度为80vh
- 优化布局使用flex确保按钮始终可见
- 文本区域自适应高度并支持滚动
- 提升大量账户导出时的用户体验

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

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

components/accounts/batch-operations.tsx

@@ -259,26 +259,26 @@ export function BatchOperations({ selectedCount, selectedAccounts, stats, onBatc
       </Dialog>
 
       <Dialog open={exportDialog} onOpenChange={setExportDialog}>
-        <DialogContent className="max-w-2xl">
+        <DialogContent className="max-w-2xl max-h-[80vh] flex flex-col">
           <DialogHeader>
             <DialogTitle>账户导出</DialogTitle>
             <DialogDescription>
               成功导出 {exportData.count} 个账户
             </DialogDescription>
           </DialogHeader>
-          <div className="space-y-4">
-            <div className="space-y-2">
+          <div className="flex-1 overflow-hidden">
+            <div className="space-y-2 h-full flex flex-col">
               <label className="text-sm font-medium">导出内容（点击文本框全选）:</label>
               <Textarea
                 value={exportData.text}
                 readOnly
                 onClick={selectAllText}
-                className="min-h-[300px] font-mono text-sm"
+                className="flex-1 font-mono text-sm resize-none"
                 placeholder="导出的账户数据将显示在这里..."
               />
             </div>
           </div>
-          <DialogFooter>
+          <DialogFooter className="mt-4">
             <Button 
               variant="outline" 
               onClick={() => setExportDialog(false)}