Fix: prevent Enter submit while IME composing (CJK)

[Decayo]

Summary

Fix: prevent Enter submit while IME composing (CJK)

Problem

• When composing text with a CJK IME (e.g., Chinese Zhuyin on macOS), pressing Enter should finalize composition, not submit the prompt.
• In the compact input bar, Enter could prematurely submit while composition was still active.

Changes

• Add explicit IME composition guards in FloatingPromptInput:
  • Track composition state via onCompositionStart / onCompositionEnd.
  • Gate Enter handling with nativeEvent.isComposing and a fallback keyCode === 229 check for environments that report 229 during composition.
• File touched: src/components/FloatingPromptInput.tsx
• No backend or dependency changes.

Behavior (after)

• Enter during composition: does not submit (finalizes IME composition).
• Enter after composition ends: submits as before.
• Shift+Enter: inserts newline (unchanged).
• Works in both compact and expanded input modes.

Risk / Scope

• Scope limited to frontend input handling in FloatingPromptInput.
• Low risk: no API, state shape, or dependency changes; only affects Enter handling when IME is active.

Test Plan

• macOS + Chinese Zhuyin IME (also applicable to other CJK IMEs):
  1. Start composing (type phonetics), press Enter → finalizes composition, does not send.
  2. After composition completes, press Enter → sends prompt.
  3. Press Shift+Enter → inserts newline.
• Verified in both compact and expanded input UIs.

Notes

• This is a common IME UX issue; the fix follows widely adopted patterns used in chat/input components.
• Maintainers can safely tweak the guard if future platforms expose better composition signals.

Checklist

• Title follows guideline prefix (Fix:)
• Change is minimal and isolated to the input component
• No new dependencies
• Manually tested locally with IME
• Unit/UI tests (N/A for this small input behavior change)