Add ai-buddy-ui plugin

[wu-clan]

Summary

• Add the official AI Buddy frontend plugin as a marketplace submodule
• Repository: https://github.com/fastapi-practices/ai-buddy-ui
• Plugin metadata: version 0.0.1, tags ai/agent/mcp

Test plan

• Submodule URL uses HTTPS
• Plugin root contains plugin.toml and README.md

[github-actions]

Plugin TOML Validation Report

Status: ✅ All Passed

Summary: 1 validated, 0 skipped

✅ ai-buddy-ui

Field	Value	Status
name	ai-buddy-ui	✅
summary	AI Buddy UI	✅
version	0.0.1	✅
description	为 AI Buddy 提供后台管理，包括供应商、模型、默认助手模型、快捷短语、助手、MCP、知识库、技能和搜索密钥	✅
author	wu-clan	✅
icon	-	➖
tags	ai, agent, mcp	✅
database	-	➖