Playground: include CSS editor in shareable URLs and URL parsing

[liannacasper]

Motivation

• Enable sharing both the Java script and the CSS editor content via playground URLs so users can reproduce full examples.
• Ensure the playground initializes from a shared URL that may contain both code and css query parameters.

Description

• Updated the README to document that shareable URLs include a css query parameter and described the URL-safe Base64 encoding for code and css values.
• Added resolveInitialCss() and cssFromUrl() to parse and decode a css parameter from the browser URL using the existing decodeSharedScript() logic.
• Changed initial state loading to call resolveInitialCss() and persist the CSS when a shared code URL is detected.
• Updated copyCurrentSourceUrl() to append an encoded css parameter when currentCss is non-empty so clipboard URLs include both editors.

Testing

• Ran the playground module tests with ./gradlew :scripts:cn1playground:common:test and the test suite completed successfully.
• Executed ./gradlew check to run the project verification tasks and they passed without errors.

---

Codex Task

[github-actions]

Cloudflare Preview

• URL: https://pr-4693-website-preview.codenameone.pages.dev
• Branch: pr-4693-website-preview