feat(admin): warn before destructive field-type switch in FormBuilder

[cdcore09]

Summary

Changing a field's type in the FormBuilder (e.g., single_choice → text) silently drops type-dependent attributes (the options array). If the user switches back, those options are lost. Currently no warning surfaces.

Requirements

• Detect when a type change will drop existing data (options, min/max, maxLength)
• Show a confirmation dialog: "Changing this field's type will discard X options. Continue?"
• Preserve the attribute union in the Coltorapps store if the user cancels

Context

Plan 4 review follow-up. v2 polish — the current behavior is intentionally destructive (server is the source of truth, attributes irrelevant to the type don't persist), but a one-way silent loss is poor UX.

Files

• apps/admin/src/components/FormBuilder.tsx (type change handler)