Improvements and fixes

[blu-Fox]

Hi, thanks for the very nice particles demo.

As a thank you, here are some quick improvements to it. 😊

• Fixed Timer interfering with Picker. This is a SwiftUI bug. It can be prevented by extracting the Picker into a separate View.
• Simplified code to update particles. No need to switch over state and have a separate text variable. All can be driven by a single state variable.
• Extracted logic into separate functions. As a result, the body is only several lines of code, and everything is neatly separated.
• Replaced GeometryReader with onGeometryChange. This is shorter and cleaner.