Feat/short answer prefill progressive disclosure

[tutntut]

Problem

Closes #9356.

On the Short Answer field editor, both pre-fill controls — "Enable pre-fill" and "Prevent pre-fill editing" — are always shown ("Prevent pre-fill editing" merely greyed out when pre-fill is off). Pre-fill is an advanced feature used by a minority of admins on a very common field, so the always-visible controls clutter the editor and push the "Create field" button out of view.

Solution

Progressive disclosure: "Prevent pre-fill editing" is hidden until "Enable pre-fill" is on, matching the house pattern in EditEmail / EditMobile. The lock value is remembered while pre-fill is off (not reset), so re-enabling pre-fill restores the prior choice.

Alternatives considered

• Remember the lock vs reset it (chosen: remember). Mirrors the Singpass authentication section, where "Collect NRIC/FIN/UENs" is hidden until authentication is enabled yet its value persists when auth is turned off. To match, the schema validator that rejected lockPrefill: true when allowPrefill: false was dropped. Safe because the lock only takes effect on the respondent side when a pre-fill value is present (isPrefillLocked = isPrefilled && lockPrefill).
• A frontend invariant helper (an earlier cut extracted/tested enforcePrefillLockInvariant) became dead code under the remember model and was removed; persistence is now covered by the backend schema spec.

Changes

• backend: remove the lockPrefill/allowPrefill cross-validator so the lock persists independently; update form_fields.schema.spec.ts.
• frontend: render the lock toggle only when allowPrefill is on; remove the old ref/.click() reset workaround.
• storybook: add a PrefillLocked story (pre-fill on, lock on).

Test plan

• Backend schema spec passes, incl. new case: lockPrefill: true persists with allowPrefill: false.
• Frontend tsc + ESLint clean.
• Manual QA: toggle pre-fill off → on → confirm the lock choice is remembered; Field ID input + copy button unchanged; respondent side unaffected.