feat: migrate compose-pay-button library to Kotlin Multiplatform (KMP)

[dmengelt]

Migrate compose-pay-button to Kotlin Multiplatform (KMP)

• Configures multiplatform targets for Android, JS, and WasmJS.
• Implements custom absolute DOM overlay positioning to support Compose Multiplatform Web targets without deprecated HtmlView.
• Resolves High-DPI screen coordinate offset alignment issues using window.devicePixelRatio.
• Adds custom buttonSizeMode config to enable full-width button scaling.
• Creates a modern, styled checkout card sample web application (:web-app) using Compose HTML/WasmJS.

[dmengelt]

fixes #200