更新 WebUI 文档：补充插件加载失败处理与热重载说明

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• Upstream PR: feat: add hot reload when failed to load plugins AstrBot#5334
• Trigger: PR merged

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AI change summary:

• 更新 zh/use/webui.md 与 en/use/webui.md，同步中英文文档。
• 在“插件加载失败处理”章节补充常见失败原因（如依赖缺失、导入错误等）。
• 新增热重载行为说明：重载插件后管理面板将自动刷新列表并反馈结果。

Please review manually before merging.

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

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