fix(web): restore AskUserQuestion picker styling

[LIUXIN557]

Summary

• restore the pre-master-merge AskUserQuestion picker density, rounded shape, shadow, and typography
• remove the question icon before the prompt text
• retain current interaction fixes, accent theme states, and composer/reasoning behavior
• add layout assertions for the restored appearance and icon removal

Verification

• targeted QuestionPicker layout, interaction, and composer architecture tests
• \pnpm test\
• \pnpm build\

The mdast patch and lockfile were used only as local validation workarounds and are not included in commit \