fix(copilot): autofocus message composer

[dcaiwei]

Summary

• keep the message composer focused when Chainlit runs in copilot mode
• preserve the existing no-autofocus behavior for the mobile web app
• add regression coverage for copilot and webapp behavior

Fixes #2944

Testing

• pnpm --dir frontend test -- MessageComposer.spec.ts
• pnpm --dir frontend type-check
• ESLint on the changed files
• Prettier check on the changed files
• git diff --check

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Summary by cubic

Fixes #2944 so the copilot message composer autofocuses even on mobile, while the web app keeps its existing mobile behavior (no autofocus). Adds regression tests for both copilot and webapp autofocus logic.

^{Written for commit 78d6003. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}