feat(admin): surface schema parse errors inline next to the offending field in FormBuilder

[cdcore09]

Summary

When the user saves a schema that the server rejects (e.g., a choice field with too many options), parseFormSchema returns { ok: false, message: "f3: too many options (max 30)" }. The FormDetailPage currently shows this in the global `actionError` banner, not inline next to the offending field.

Requirements

• Parse the error message field-id prefix (e.g., `f3:`) and pass the per-field error down to FormBuilder
• Highlight the offending field with a red border + inline error message
• Banner stays for non-field-specific errors

Context

Plan 4 review follow-up. UX polish — current behavior works but is hard to act on for forms with many fields.

Files

• `apps/admin/src/pages/forms/FormDetailPage.tsx` (Save schema handler)
• `apps/admin/src/components/FormBuilder.tsx` (accept and render field-specific error prop)