chore(react): migrate library build from Rollup to Vite (lib mode)

[anastasialanz]

Summary

Replace the Rollup-based build of @deque/cauldron-react with Vite's library mode. packages/react is the only package that would change — packages/styles (postcss-cli), the docs site (webpack), and Playwright CT (already on Vite) stay as-is.

The migration must preserve the published contract:

• main: lib/index.js (CJS), types: lib/index.d.ts, style: lib/cauldron.css
• Code-split chunks for the icon SVGs (driven by import(`./icons/${iconName}.svg`) in packages/react/src/components/Icon/index.tsx)
• Externals: dependencies + peerDependencies + /^react-syntax-highlighter/
• SVGR svgo config: removeViewBox: false, removeDimensions, force width=24/height=24
• process.env.NODE_ENV left as a runtime reference (Icon's test-mode guard depends on it)

Proposed approach

• Add packages/react/vite.config.ts using build.lib + @vitejs/plugin-react + vite-plugin-svgr + vite-plugin-dts
• Swap devDependencies: drop rollup, @rollup/plugin-typescript, @rollup/plugin-commonjs, @rollup/plugin-dynamic-import-vars, @svgr/rollup; add vite, @vitejs/plugin-react, vite-plugin-svgr, vite-plugin-dts
• Update scripts: build:lib → vite build, dev → vite build --watch (CSS step unchanged)
• Delete packages/react/rollup.config.js after parity is verified
• Out of scope: Jest setup, .babelrc, postcss.config.js, the docs-site webpack config

Migration risks

1. Icon dynamic imports — Vite ships @rollup/plugin-dynamic-import-vars internally; verify per-icon chunks are still emitted
2. SVGR output parity (both plugins wrap @svgr/core; spot-check one icon)
3. process.env.NODE_ENV must not be inlined in the lib build
4. .d.ts emission via vite-plugin-dts instead of @rollup/plugin-typescript
5. The /^react-syntax-highlighter/ external regex must be preserved exactly

Verification plan

• yarn --cwd=packages/react build produces lib/ with parity to the previous Rollup output (diff -r the trees; bundle size within ~5%)
• lib/ still contains process.env.NODE_ENV as a runtime reference
• lib/index.d.ts and per-component .d.ts files are present and type-check
• Docs site (yarn dev from repo root) renders icons correctly against the new lib/
• yarn --cwd=packages/react test passes (tests don't go through the bundler, so should be unaffected)
• Playwright CT screenshots (yarn screenshots) catch no visual regressions
• npm pack --dry-run in packages/react matches today's published file list