[registration] implement faculty → major/specialisation cascading selection

[WilliamTayNZ]

Important: When you begin this ticket, create a new branch from feat/registration-rework and not main. When you open a PR, open it into feat/registration-rework and not main.

Description

Faculty selection is being capped at 2, and the current "other faculty" free-text field is being replaced by a separate majors section where users can type up to 4 major/specialisation entries. Faculties and majors are independent — majors are not tied to specific faculties.

This replaces the current approach where faculty is an uncapped multi-select checkbox group with an "other" option and free-text fallback.

Requirements

• Cap faculty checkbox selection at 2 (must work without JS — server-side validation in actions.ts rejecting more than 2, with a clear error message; optional client-side JS enhancement to disable remaining checkboxes once 2 are selected)
• Remove the "Other" faculty checkbox and otherFaculty free-text field — custom faculties are no longer supported; the fixed checkbox list is the complete set
• Add a majors/specialisation section: up to 4 free-text inputs where the user types their major(s)
• Majors are not faculty-specific — they're a flat list, independent of which faculties are checked
• Each major entry should respect MAX_LENGTHS (MAX_LENGTHS.major = 40 in constants.ts)
• Remove the otherFaculty merge logic from actions.ts's case "final" (mergedPrev block) — no longer needed since "other" is gone
• Remove otherFaculty from RegistrationDraft type, stripIrrelevantFields, and the NewUoaPage component (the useRef + onInput auto-check enhancement is also removed, since it was specifically for the "Other" checkbox)
• Update actions.ts's case "newUoa" validation for the new constraints (max 2 faculties, max 4 majors, no otherFaculty)
• Update toParsedSubmission to map majors from the draft to ParsedRegistrationFormSubmission

Acceptance Criteria

• Faculty selection is limited to 2 (enforced in server actions)
• "Other" faculty option and otherFaculty field are removed from the UI and from actions.ts
• Up to 4 major/specialisation text inputs are available
• Submitting more than 2 faculties or more than 4 majors returns a clear validation error
• The mergedPrev / otherFaculty merge logic in case "final" is removed
• Core functionality works without JavaScript
• No-JS path: all major fields submit as standard form inputs via POST

Notes

Does not depend on ticket #83 for form UI or server action work - the page components, actions.ts validation, and draft cookie flow
can all be built and tested independently. Ticket #83 's schema changes are only needed for end-to-end persistence (the final submission writing to Postgres), which is wired in ticket #87.

For no-JS, the "add another major" interaction can use the same server-round-trip pattern as back-navigation: a submit button with
intent="addMajor" increments a majorCount in the cookie draft and re-renders the page with one more input. Starts at 1, caps at MAX_MAJORS.