fix: restore app nav create submenu interaction

[hjlarry]

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Summary

fix #35680

Fix the app navigation "New App" submenu interaction.

The previous implementation nested a Headless UI Menu inside another MenuItems. With the current Headless UI behavior, clicking the nested MenuButton could close the parent menu before the submenu interaction completed, so the submenu appeared unresponsive.

This change replaces the nested menu with a locally controlled inline submenu inside the parent menu. It also removes the default focus outline from the parent menu panel.

Screenshots

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods