bug: question custom answer textarea grows unboundedly, hiding submit button in web mode

[chx9]

Description

In serve/web mode, when the AI uses the question tool with multiple-choice options that include a "Type your own answer" field, entering a large amount of text causes the textarea to grow without bound. This pushes the Submit/Next/Dismiss buttons outside the visible area, making them inaccessible.

Steps to reproduce

1. Run opencode in serve mode (opencode serve)
2. Open the web UI
3. Trigger a question with options (e.g. ask the model to use the question tool)
4. Click "Type your own answer"
5. Paste or type a large block of text (several lines)

Expected behavior

The textarea should stop growing after a reasonable height and scroll internally. The footer with Submit/Next/Dismiss buttons should always remain visible.

Actual behavior

The textarea expands to fit all content with no height limit, pushing the footer buttons out of the viewport.
as in the pic below, can't find the submit button

Root cause

• resizeInput() in session-question-dock.tsx sets el.style.height = el.scrollHeight with no upper bound
• question-custom-input CSS uses overflow: hidden, so content expands the element instead of scrolling internally
• No max-height is set on the textarea

Environment

• Web mode (serve)