更新 WebUI 文档：插件失败处理与市场 UX 改进

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Generated automatically by astrbot-docs-agent-server.

• Upstream PR: feat: improve plugin failure handling and extension list UX AstrBot#5535
• Trigger: PR merged

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AI change summary:

• 同步更新中英文文档 (zh/use/webui.md, en/use/webui.md)。
• 在“已安装插件”文档中新增“加载失败插件”区域说明，支持重载、卸载及数据清理操作。
• 更新“插件市场”文档，补充随机推荐插件的显示/隐藏逻辑及搜索框位置调整说明。

Please review manually before merging.

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Experimental bot notice:

• This output is generated by AstrBot-Doc-Agent for review only.
• It does not represent the final documentation form.

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