implement Plunk email driver with comprehensive tests

- Add plunkEmail driver following IEmailDriver interface
- Support single and multiple recipients (up to 5 max per Plunk API)
- Handle both string and { text, html } body formats
- Include sender customization (from, name, reply)
- Add recipient validation and error handling
- Create comprehensive unit tests with 6 test cases
- Export plunkEmail from core drivers index
- Update task list with implementation progress

app/src/core/drivers/index.ts

@@ -5,3 +5,4 @@ export type { IEmailDriver } from "./email";
 export { resendEmail } from "./email/resend";
 export { sesEmail } from "./email/ses";
 export { mailchannelsEmail } from "./email/mailchannels";
+export { plunkEmail } from "./email/plunk";