Add auto-focus to text input when window regains focus

[nelsonblaha]

Summary

Automatically return cursor focus to the text input area whenever the browser window or tab regains focus.

Motivation

This improves UX by allowing users to start typing immediately after switching back to the app without manually clicking the input field. This is especially useful when:

• Switching between tabs to reference documentation
• Returning from another application
• Using the app alongside other tools

Changes

• Added a window focus event listener in AgentInput.tsx
• Only runs on web platform (mobile has different focus behavior)
• Uses existing inputRef to restore focus programmatically
• Properly cleans up event listener on unmount

Testing

Tested on web:

• ✅ Focus returns to input when switching browser tabs
• ✅ Focus returns when switching back from another application
• ✅ No impact on mobile platforms
• ✅ Event listener properly cleaned up on unmount

Implementation Details

The implementation is minimal and follows existing patterns in the codebase:

• Uses React's useEffect hook with proper cleanup
• Only activates on Platform.OS === 'web'
• Calls the existing inputRef.current?.focus() API