feat: registration form field display_width and option descriptions

Add configurable column widths (full/half) and optional descriptions
for radio/select/checkbox options on registration form fields.

- Migration adds display_width column to both tables
- FieldDisplayWidth enum with smart defaults per field type
- normalized_options accessor for backwards-compatible option format
- Portal form renderer uses display_width for VRow/VCol grid layout
- Radio/select/checkbox options render with descriptions
- Admin field editor supports display_width toggle and description input
- System templates updated with appropriate widths and descriptions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev-docs/API.md

@@ -711,6 +711,14 @@ Creates a COPY of the template as an event field. The copy is independent — ch
 
 Copies all `registration_form_fields` from the source event. Source must belong to the same organisation. Existing fields on the target event are kept.
 
+### Response Fields
+
+Each registration form field response includes:
+
+- `options` — raw stored format (string array or object array, for backwards compatibility)
+- `normalized_options` — always `[{label, description}]` format (null when field has no options). Descriptions are null when not set. Use this for rendering.
+- `display_width` — `"full"` or `"half"`, controls form layout column width. Auto-set based on field type when not explicitly provided.
+
 ### Tag Picker Fields
 
 For `tag_picker` fields: the API response includes `available_tags` array (from `person_tags`, filtered by `tag_category` if set) so the frontend knows which tags to render as options.