fix: prevent Enter from sending during IME composition on macOS

[QZQ-go]

PR Title

fix: prevent Enter from sending during IME composition on macOS

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Summary

• Fixes issue Enter key sends message instead of dismissing IME candidate box #70 where pressing Enter while Chinese IME candidate box is visible could send the message prematurely on macOS.
• Adds stronger IME composition detection in chat input send handling.
• Adds a regression E2E scenario to verify Enter is ignored during composition and works after composition ends.

Problem

On macOS Chinese IME (system/Sogou/Tencent), Enter used for candidate confirmation could trigger message send before composition was truly finished.

Root Cause

Send logic depended mainly on KeyboardEvent.isComposing, which can be transient/insufficient around composition end timing in some IME flows.

Fix Details

1. ChatInput.tsx

   • Track composition lifecycle via refs (isImeComposingRef).
   • Add short guard window after composition end (IME_ENTER_GUARD_MS = 120) to avoid accidental Enter send.
   • Block Enter send if composing or just finished composition.

2. RichTextInput.tsx

   • Expose onCompositionStart / onCompositionEnd callbacks.
   • Use combined composition check (native isComposing || internal composing ref) in key handling.

3. E2E

   • Add composition event helpers in chat input page object.
   • Add regression test in basic-chat.spec.ts:
     • Enter during composition should NOT send.
     • Enter after composition end should send.

Validation

• ✅ Manual verification on macOS with Chinese IME: passed (report from local testing).
• ⚠️ npm run type-check / npm run build in src/web-ui: currently fail due to existing repository-wide TS baseline issues unrelated to this PR.
• ⚠️ tests/e2e npm run test:chat: blocked in local environment by tauri-driver/config startup issues.

Changed Files

• src/web-ui/src/flow_chat/components/ChatInput.tsx
• src/web-ui/src/flow_chat/components/RichTextInput.tsx
• tests/e2e/page-objects/components/ChatInput.ts
• tests/e2e/specs/chat/basic-chat.spec.ts

Related

• Closes Enter key sends message instead of dismissing IME candidate box #70