fix: respect server default certification domain

[SonJH7]

Summary

• avoid appending the default school domain on the client when sending certification requests so the backend can handle it
• keep validating custom domains while omitting the domain field when the default domain is assumed
• stop flagging local-part-only entries as invalid in the email input

Testing

• npm run test -- src/tests/certification/EmailCertPage.test.tsx
• npm run test -- src/tests/api/certification.requestEmail.test.ts

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https://chatgpt.com/codex/tasks/task_e_690cd3c4f70c83328ee5022aeff87c15