Integrate Theme config

[AustinChangLinksys]

• Investigate theme config json with UI kit.
• Integrate with jenkins and codebase

[PeterJhongLinksys]

Completed theme configuration refactoring.

Summary of changes:

• Refactored ThemeConfigLoader: Converted from a static class to an instance class, now serving as the single source of truth for theme resolution. Added forTesting() constructor to support dependency injection.
• New themeConfigProvider: Replaced the old deviceThemeConfigProvider with a clean Riverpod provider that delegates logic to ThemeConfigLoader.
• Consolidated Theme Logic: Moved device theme resolution (BrandUtils.getDeviceTheme) and model mapping logic into ThemeConfigLoader.
• Cleaned up Consumers: Updated app.dart, main.dart, and demo entry points to use the new provider. Demo apps no longer manually call the loader.
• Documentation: Added comprehensive architecture documentation in doc/theme/theme_architecture.md.
• Verified: All tests passed (3117/3117), including new unit and integration tests for the refactored components.