fix: prevent Enter from sending message during IME composition

[baryhuang]

Summary

• When typing Chinese/Japanese/Korean using an IME, pressing Enter to confirm a character selection was incorrectly triggering message send
• Added isComposing guard at the top of handleKeyDown in all 3 chat input components: workspace frontend, packages/go, and studio MessageInput
• Uses the standard KeyboardEvent.isComposing property with a Process key fallback for legacy browsers

Test plan

• Tested locally with Chinese IME input — Enter confirms character without sending
• Verify normal Enter-to-send still works without IME active
• Verify Shift+Enter newline still works
• Verify @mention selection with Enter still works (without IME)

🤖 Generated with Claude Code

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