Fix light selector in config_flow

[Snuffy2]

This pull request refactors error handling and validation logic in the animated_scenes configuration flow, improving consistency and user feedback. It also updates the schema for selecting lights to ensure compatibility with Home Assistant's platform constants. The most important changes are grouped below:

Validation Improvements:

• Adds a check to ensure that at least one light is selected during scene configuration, providing clear error feedback if none are chosen. [1] [2] [3]

Schema Updates:

• Updates the light selector schema to use Platform.LIGHT for domain specification and sets the default to an empty list, improving compatibility and preventing schema errors. [1] [2]

Other Minor Fixes:

• Ensures default values are correctly set and schema extension logic is streamlined for better maintainability. [1] [2]

These changes collectively improve the reliability, maintainability, and user experience of the configuration flow for animated scenes.

Fixes #54