fix(client): declare markdown-it-github-alerts as a dependency

[exKAZUu]

Description

packages/client/styles/index.ts imports the stylesheets of markdown-it-github-alerts (since #2547):

import 'markdown-it-github-alerts/styles/github-colors-light.css'
import 'markdown-it-github-alerts/styles/github-colors-dark-media.css'
import 'markdown-it-github-alerts/styles/github-base.css'

but @slidev/client only gets the package through hoisting: it is declared in packages/slidev/package.json, not in packages/client/package.json. With a package manager layout that does not hoist transitive dependencies next to @slidev/client (e.g. Bun's isolated linker with its global store, where each package can only see its declared dependencies), slidev build fails with:

Error: [vite]: Rolldown failed to resolve import "markdown-it-github-alerts/styles/github-colors-light.css" from ".../node_modules/@slidev/client/styles/index.ts".

This PR declares markdown-it-github-alerts in @slidev/client via the existing catalog:prod entry, so the import resolves regardless of hoisting. pnpm verify passes locally.

Linked Issues

Related to #2547 (which introduced the import).

Additional context

Reproduced with @slidev/cli@52.19.0 installed by Bun 1.4.0 with linker = "isolated" and globalStore = true.

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