vscode: QuestionDialog lacks multi-select support; consider a single-form layout

[wbxl2000]

Problem

The VS Code webview QuestionDialog (apps/vscode/webview-ui/src/components/QuestionDialog.tsx) treats every question as single-select: clicking an option immediately advances to the next question or submits. When the agent sends a question with multi_select: true (allowed per-question by the AskUserQuestion schema), the user can only pick one option and the agent receives just the first clicked label instead of the full selection.

This is a pre-existing gap, not introduced by #2326 — before that PR only the first question was shown at all, with the same single-select behavior. Surfaced by a review comment on #2326.

Two related UX gaps in the current step-through design:

• No way to go back and change a previous answer.
• No skip: users must answer every question (or abort the whole run), even though partial answers are contract-legal — only an all-empty submission counts as dismissed.

Proposal

Render all questions (the schema caps at 4) as one scrollable form instead of a step-through wizard:

• Radio semantics for single-select questions, checkboxes for multi-select questions.
• One explicit Submit at the bottom; the form itself serves as the review.
• Allow submitting with unanswered questions (show a hint, don't block); an all-empty submission keeps the current dismiss semantics.

Multi-select answer format must match the TUI convention: selected labels joined with ', ' (see updateAnswer in apps/kimi-code/src/tui/components/dialogs/question-dialog.ts). Answers stay keyed by question text; uniqueness is enforced tool-side.

Refs: #2299, #2326