feat: Group name for Code Groups to sync opened tab

[juji]

Description

This PR adds support for named code groups with automatic tab synchronization and localStorage persistence. When multiple code groups share the same name parameter, selecting a tab in one group automatically selects the corresponding tab in all other groups with the same name, and the preference is saved across page visits.

What this PR adds:

• New name parameter for code-group containers
• Tab synchronization across groups with matching names
• localStorage persistence - tab selections are remembered across page visits and sessions
• Client-side logic to sync tab selections in real-time
• Data attribute (data-group-name) added to HTML output
• Validation to ensure group names only contain alphanumeric characters, hyphens, and underscores

Use case:
This is especially useful in documentation where the same tool (e.g., package managers like npm/pnpm/yarn) is shown in multiple contexts throughout a page, providing a consistent and improved user experience. Users only need to select their preference once, and it persists across the entire documentation site.

Additional Context

Implementation highlights:

• Markdown syntax: ::: code-group name=package-managers
• Tab synchronization is automatic and requires no user configuration
• User preferences are stored in localStorage under vitepress:tabsCache
• Preferences persist across page visits and are restored on load
• Non-breaking change - the parameter is optional
• Includes comprehensive E2E tests for functionality and edge cases
• Documentation updated with interactive examples demonstrating the sync behavior

Files changed:

• src/node/markdown/plugins/containers.ts - Parsing and HTML generation
• src/client/app/composables/codeGroups.ts - Client-side synchronization logic and localStorage persistence
• __tests__/e2e/markdown-extensions/ - E2E tests and fixtures
• docs/*/guide/markdown.md - User documentation in all languages

Testing:

• All existing tests pass (40 E2E tests)
• New E2E tests verify tab synchronization works correctly
• Edge cases tested (invalid characters, spaces, empty values)
• localStorage persistence tested for correct restoration
• Live examples in documentation demonstrate the feature

Translation needed:

• English documentation is complete in docs/en/guide/markdown.md
• All other language versions (es, ja, ko, zh, pt, ru, fa) include English content with <!-- TODO: Translate to [Language] --> markers
• I am not capable of translating to these languages and would appreciate help from native speakers or the community
• The English content is ready to be translated and clearly marked in each file

---

Code Example

::: code-group name=package-managers

```bash [npm]
npm install vitepress
```

```bash [pnpm]
pnpm add vitepress
```

```bash [yarn]
yarn add vitepress
```

:::

<!-- Later in the same domain: -->

::: code-group name=package-managers

```bash [npm]
npm run docs
```

```bash [pnpm]
pnpm run docs
```

```bash [yarn]
yarn docs
```

:::

Screen.Recording.2025-11-02.at.23.57.47.mov

Showing how it works. Also shows that on refresh, the last selected tab is still ON

---

Tip

The author of this PR can publish a preview release by commenting /publish below.

[juji]

doc changes:

https://deploy-preview-5012--vitepress-docs.netlify.app/guide/markdown#named-code-groups

need help for other langs