fix(app): ignore IME composition keys in question dock custom input

[sawa-zen]

Issue for this PR

Closes #49154

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

In the Desktop question dock (SessionQuestionDock), the custom answer textarea treated Enter/Escape pressed to confirm or cancel IME composition (e.g. Japanese kana-kanji conversion) as commit/dismiss, moving focus out to the option card.

Applies the same isImeComposing pattern used in components/prompt-input.tsx (event.isComposing || composing() || event.keyCode === 229):

• textarea onKeyDown: early return while IME is composing, before Escape/Enter handling
• Track composition via onCompositionStart / onCompositionEnd
• nav (DockPrompt-level keydown): skip reject() on Escape and next() on Cmd/Ctrl+Enter while IME is composing

How did you verify your code works?

Checked against the screen recording in #49154 (focus leaving the inner textbox for the outer option card after conversion confirm). Full app typecheck not run in this environment (sparse checkout, no installed workspace deps); the change follows the existing pattern in prompt-input.tsx.

Screenshots / recordings

See the recording attached to #49154. No new recording attached.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found one potentially related PR:

PR #44826: fix(prompt-input): keep the editor inert during IME composition
#44826

This PR is related because it also addresses IME composition handling in the prompt-input component. The current PR (49164) applies the same isComposing pattern from #44826 to the question dock's custom input textarea. Both PRs focus on preventing IME composition keys from being processed as regular input events.

[github-actions]

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