Add App component with channel-based reactive integration

The App component demonstrates the complete reactive counter example:
- Creates a bidirectional channel (buffered, capacity 10)
- Input field sends values to the channel on input events
- Counter component receives updates via the channel
- Non-blocking channel send using select/default pattern

This component is manually written because the parser doesn't yet support
type arguments in function calls like make(chan int, 10). Future work will
extend the parser to support this syntax.

The App component shows the full pattern:
1. Parent creates channel
2. Parent passes channel to child via WithChannel option
3. Child's BindApp() starts goroutine listener
4. Goroutine updates current value and triggers re-render
5. Child's Render() uses non-blocking current value

Author: claude