feat: add DOM-safe client entrypoint

[Akin01]

Summary

• add explicit @akin01/solid-email/client export for DOM/CSR preview components
• keep the package root SSR/email-rendering oriented with render/compile intact under browser conditions
• exclude render, compile, and Tailwind from the client entrypoint and document the split

Closes #8.

Verification

• pnpm --filter @akin01/solid-email run test
• pnpm test:e2e
• pnpm typecheck
• pnpm lint
• pnpm --filter @akin01/solid-email run build