Add tsdown for cjs and esm builds

[abhithesys]

What

Migrate all publishable packages to dual ESM/CJS builds using tsdown, eliminating the need for transpilePackages in Next.js and build.transpile in Nuxt. This fixes the friction reported in #360 where consumers had to manually configure their bundlers to use OpenUI packages.

fixes point 3 in #360.
fixes: #436

Changes

Build system

• Migrated lang-core, react-lang, react-headless, and react-email builds from tsc to tsdown with dual ESM (.mjs) + CJS (.cjs) output and bundled type declarations (.d.mts, .d.cts)
• Added CJS and ESM bundles for react-ui (via tsdown) for the . and ./genui-lib entry points, alongside the existing tsc-based ESM build for per-component sub-paths
• tsdown generates bundled .d.mts/.d.cts declarations (no bare relative imports), making types resolve correctly in all TypeScript moduleResolution modes

Package exports

• Updated exports maps across all packages with proper import/require conditional exports, each pointing to the correct .mjs/.cjs JS and .d.mts/.d.cts type files
• Added exports field and "type": "module" to react-headless (previously had no exports map)
• Updated top-level main, module, and types fields to reference the correct tsdown-generated artifacts
• Removed ./tailwind export from react-ui (unused)
• Moved zod from dependencies to peerDependencies across all packages that use it

Publish guards

• Added check:publint and check:attw scripts to all packages
• Added prepublishOnly to all packages running check:publint && check:attw before every publish, preventing accidental publishing of a broken build
• Added lang-core to the GitHub Actions publish workflow (was missing)

Quality fixes

• Fixed Theme re-export in react-ui/src/index.ts to use type modifier, resolving a rolldown MISSING_EXPORT build warning
• Removed redundant shareThread.scss import from ShareThreadModal.tsx (already forwarded via openUIChat.scss)
• Removed transpilePackages from all Next.js example and docs configs

Versions

All packages receive a minor version bump:

Package	Old	New
@openuidev/react-ui	0.10.2	0.11.0
@openuidev/lang-core	0.1.2	0.2.0
@openuidev/react-lang	0.1.5	0.2.0
@openuidev/react-headless	0.7.11	0.8.0
@openuidev/react-email	0.1.3	0.2.0

Test Plan

• Built all packages with pnpm build and confirmed dual-format output files exist (.mjs, .cjs, .d.mts, .d.cts)
• Ran pnpm check:publint — no errors on all packages (cosmetic repository URL suggestion only on react-ui)
• Ran pnpm check:attw — all green (node10, node16 CJS, node16 ESM, bundler) on all packages
• TypeScript consumer type-check: imported Button, Theme, and openuiLibrary from the built .d.mts declarations with strict mode — no errors
• CJS runtime: require() on lang-core, react-headless, and react-ui/Button — exports load correctly
• Visual integration test: created a Vite React app inside the monorepo referencing all packages via workspace:^, confirmed Button, Callout, ThemeProvider, and openuiLibrary render correctly in the browser and pnpm build succeeds
• Verified Next.js examples (openui-chat, shadcn-chat, vercel-ai-chat) build successfully without transpilePackages