fix(ui): wire up ER diagram help button to open docs (#1695)

[HandSonic]

Problem

ER 图工具栏的帮助按钮 onClick={() => {}} 是空函数，点击无反应。

Fix

将帮助按钮的 onClick 改为调用 openWebPage() 打开 Chat2DB 文档页面，与项目中其他帮助入口行为一致。

Related

Fixes #1695

[openai0229]

The button wiring is directionally correct, but please avoid hardcoding https://docs.chat2db-ai.com/.... Chat2DB already has the region- and edition-aware appUrlConfig.DOCS_URL, which selects the appropriate chat2db.ai or chat2db-ai.com host and the current /resources route. Hardcoding this URL bypasses that selection and points to an outdated or currently unavailable route. Please read DOCS_URL from the global app configuration and pass it to openWebPage(). If an ER diagram-specific help page is available, that would be preferable to the generic getting-started page.

[HandSonic]

Fixed. Removed the hardcoded URL and now reads appUrlConfig.DOCS_URL from the global store via useGlobalStore, which automatically selects the correct domain (chat2db.ai or chat2db-ai.com) based on region.