feat(registration): cap faculty selection and replace other faculty with majors

[FinleyNeilson]

Summary

• Cap faculty checkbox selection at MAX_FACULTIES (2), enforced server-side in actions.ts
• Remove the "Other" faculty checkbox and otherFaculty free-text field entirely (UI, actions.ts, RegistrationDraft, stripIrrelevantFields)
• Add an independent majors/specialisation section: up to MAX_MAJORS (4) free-text inputs, each capped at MAX_LENGTHS.major (40 chars)
• No-JS "add another major" via a new addMajor intent that round-trips through the cookie draft, incrementing majorCount and preserving in-progress field values — mirrors the existing "back" navigation pattern
• Map majors through to ParsedRegistrationFormSubmission via toParsedSubmission

Closes #85