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

[sawa-zen]

Fixes #49154.

In the Desktop question dock (SessionQuestionDock), the custom answer textarea treated every Enter/Escape keydown as commit/dismiss — including the keystrokes used to confirm or cancel IME composition (e.g. Japanese kana-kanji conversion with Google Japanese Input). As a result, confirming a conversion moved focus out of the input to the option card, interrupting continuous Japanese input.

This mirrors the existing isImeComposing pattern from components/prompt-input.tsx (event.isComposing || composing() || event.keyCode === 229, with explicit composition tracking for platforms where compositionend fires before the confirming Enter):

• textarea onKeyDown: return early while IME is composing, before the Escape/Enter handling.
• Track composition via onCompositionStart/onCompositionEnd.
• nav (DockPrompt-level keydown): don't reject() on Escape or advance on Cmd/Ctrl+Enter while IME is composing (keydowns bubble up from the textarea).

Verified against the screen recording attached to #49154 (focus leaving the inner textbox for the outer option card after conversion confirm). I could not run the full app typecheck in this environment (sparse checkout, no installed workspace deps); the change follows the exact pattern already used in prompt-input.tsx.

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