feat(theme)!: add layer for base.css to support tailwind v4

[SoonIter]

Summary

This is a breaking change for compatibility

https://caniuse.com/?search=%40layer

Related Issue

#2160

vuejs/vitepress#4904

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR adds support for Tailwind CSS v4 by reorganizing CSS layer structure to ensure proper CSS cascade ordering. The changes wrap base styles in a CSS layer and extract Tailwind directives to a separate file to maintain compatibility with the new Tailwind architecture.

• Wraps base styles in a CSS layer (rp-base) for better cascade control
• Separates Tailwind directives into a dedicated stylesheet
• Updates import order to load the new Tailwind CSS file

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/theme-default/src/styles/tailwind.css	New file containing extracted Tailwind directives for components and utilities
packages/theme-default/src/styles/index.ts	Adds import for the new tailwind.css file
packages/theme-default/src/styles/base.css	Wraps styles in CSS layer and removes Tailwind directives

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