fix: support single-question multiSelect in AskUserQuestion

[NOirBRight]

Problem

When AskUserQuestion has a single question with multiSelect: true, clicking an option immediately sends the answer as a single choice — the same behavior as a single-question single-select. The multiSelect property is completely ignored.

Also, the "Type something." free-text input row that pi TUI always shows for single-select questions was missing in pi-web entirely, and multi-select questions with free-text options (like "其他"/"Other"/"自定义"/"Custom") rendered them as regular buttons instead of text inputs.

Root Cause

Three layers all had the same bug:

1. Rendering: data-multiple attribute and ask-question-multiselect CSS class were never emitted for question blocks, so the UI had no way to distinguish multi-select from single-select.

2. Click handling: chat-composer-runner.js used questionCount === 1 as the sole condition for immediate-send, without checking the multiSelect / data-multiple flag.

3. Export renderer: The standalone export page (internal/ui/export/app/50-render-entry.js) had the same rendering omission.

Additionally, none of the renderers emitted a free-text input row, and there was no mechanism to detect free-text option labels.

Fix

Multi-select rendering (3 files)

• web/src/session/render/session-entry-renderer.js
• web/src/session/live/live-renderer.js
• internal/ui/export/app/50-render-entry.js

Each now checks q.multiSelect === true alongside question count:

• isMulti = questions.length > 1 || questions.some(q => q.multiSelect === true) (for card-level)
• qMultiple = q.multiSelect === true (for per-question data-multiple attr)
• Adds ask-question-multiselect class to multi-select option elements

Click handling (chat-composer-runner.js)

• Condition for immediate-send changed from questionCount === 1 to questionCount === 1 && !qMultiple
• Multi-select toggle: option.classList.toggle("selected")
• Multi-select submit: collects all .selected options, joins with comma
• Multi-select question blocks also get a "Send answers" button

Free-text input row (same 3 renderers + chat-composer-runner)

• Single-select: always shows a text input at the bottom of options
• Multi-select: shows a text input when an option label matches free-text heuristics:
  • "Type something." (pi standard)
  • Labels containing "其他", "Other", "自定义", "Custom", "自行输入", "自由输入", "enter your"
• Free-text option labels are hidden from the button list and rendered as a dashed-border <input> instead
• Enter key in the freetext input submits the answer (single-select sends immediately; multi-select collects with other selected options)
• data-freetext="true" attribute on question blocks that have a freetext input

Styling (session.css)

• .ask-question-option.multiselect — dashed border, ☐/☑ indicators
• .ask-question-freetext-input — dashed border, accent focus color

Tests

• session-entry-renderer.test.js — 2 new test cases for data-multiple and ask-question-multiselect
• live-renderer.test.js — 1 new test case for multiSelect rendering
• chat-composer-runner.test.js — 4 new test cases for multi-select toggle, submit, single-select guard
• ask_user_question_render_test.go — Go embedded asset test checks for data-multiple and ask-question-multiselect

Testing

• All frontend tests pass
• All Go tests pass
• Manual browser verification: single-select shows freetext input, multi-select with "其他" shows freetext input, regular multi-select shows Submit button