Restore CSS theme and add demo tests

[shai-almog]

Summary

• replace the single hello-world screen with a demo browser and sample demos
• add shared demo interfaces and registry utilities so new demos can plug in easily
• update docs CI to build the demos with Maven so changes are tested automatically
• restore the CSS theme configuration and add component identifiers to the demo UI
• add automated tests covering the demo registry, browser, and sample demos
• ensure codename1.cssTheme=true remains defined in the demo project settings

Testing

• mvn -B -ntp -Dgenerate-gui-sources-done=true -pl common -am -f docs/demos/pom.xml test (fails: network is unreachable while downloading UpdateCodenameOne.jar)

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https://chatgpt.com/codex/tasks/task_e_68fa3a2c81ec833186721a889a1397ff

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF