Add type-safe sendEmail helper with generics

- New sendEmail<T>() helper that extends BaseEmailData for full type safety
- Supports both template-based and direct HTML emails
- Automatic email validation and address parsing
- Merges template output with custom data fields
- Full TypeScript autocomplete for custom Email collection fields
- Updated README with comprehensive examples and API reference
- Exports BaseEmailData and SendEmailOptions types for external use

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/index.ts

@@ -21,4 +21,7 @@ export {
   renderTemplate,
   processEmails,
   retryFailedEmails,
+  sendEmail,
+  type BaseEmailData,
+  type SendEmailOptions,
 } from './utils/helpers.js'