Fix onboarding question card styling side-effects

The onboarding page deliberately restyles question cards into full-width tiles, but the page-scoped `:deep` overrides in `Onboarding.vue` (`.ff-radio-btn` / `.ff-checkbox` set to `display: flex; width: 100%`) also stretch the selection controls themselves. The cards should keep using the same Expert question UI as the chat drawer, just at full chat width; the checkboxes and radios keep their normal size.

Side-effects to fix:

- Single-select options render the description on the same line as the label (the override turns `ff-radio-group`'s label plus `p.ff-description` into a flex row). Multi-select stacks correctly.
- The free-text option's input is crushed to a few px because the control becomes a full-width tile.

Done when: question cards span the full chat width with normally sized controls, single-select descriptions render under their label, the free-text input has full usable width, and the drawer chat rendering is unchanged.