fix(app): skip editor reconcile during IME composition

[ysm-dev]

Fixes #14371
Also related: #15506

What

The contenteditable prompt editor re-renders (reconciles) the DOM whenever the prompt state updates. When IME composition is active (e.g. typing Korean after Shift+Enter), this reconcile destroys the in-progress composition node, causing the first consonant to split off and duplicate to the right of the cursor.

Change

• Guard the createEffect that syncs prompt state → DOM so it skips reconcile while composing() is true
• On compositionend, schedule a single deferred reconcile via requestAnimationFrame (bails out if a new composition started in between)
• Extract the reconcile logic into a shared reconcile() so both the effect and compositionend use the same path

No new dependencies, no behavior change for non-IME input.

Verification

Tested locally on macOS desktop (Tauri) with Korean IME:

1. Type Korean text → Shift+Enter → type ㅎ then ㅏ → correctly produces 하 (no duplicate consonant)
2. Normal Enter still submits
3. @mention / slash command popovers still work during typing
4. bun run typecheck and bun test pass in packages/app

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code written by gpt-5.4

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