Prevent IME composition from submitting chat

[Maklu]

What does this change?

Prevents the chat composer from treating Enter and other keyboard shortcuts as commands while an IME composition is active. This fixes the existing bug reported in #81, where confirming a Chinese, Japanese, or Korean candidate could submit the message.

The guard covers both the standard KeyboardEvent.isComposing signal and the legacy process-key value (keyCode === 229). Focused tests cover composition Enter, legacy process-key Enter, and ordinary Enter.

Why is this obviously correct and trivially verifiable?

The complete behavior is contained in one pure predicate and one early return in the existing keydown handler. When composition is active, the handler does nothing and leaves the event to the IME. When it is not active, execution continues through the unchanged keyboard handling path. The regression tests exercise both branches directly.

Validation

• pnpm --filter @gadgets/workshop-frontend exec tsc --noEmit
• pnpm --filter @gadgets/workshop-frontend test (29 files, 153 tests passed)

Checklist

• This is a small, concrete change; it is not a feature, refactor, or low-value cleanup.
• I understand that maintainers decide whether the change is obviously correct and trivially verifiable.
• I have read and followed the contribution guidelines.

Closes #81

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[Maklu]

Closing this because it overlaps with #140, which was opened earlier and addresses the same IME composition / Enter submission bug. The full scope remains tracked in #81. Apologies for the duplicate, and thanks for taking a look.