Implement Custom Theme Configuration System

[sebastianwalenta]

I have implemented a comprehensive theme configuration system for the Flutter application.

Key features include:

1. Dynamic Theming: Users can now change the primary theme color and global text size multiplier directly from the Settings screen.
2. Persistence: Theme selections are saved using SharedPreferences and restored automatically when the app restarts.
3. Clean Architecture: Introduced a ThemeConfig model and ConfigProvider service using the provider package, separating theme logic from UI components.
4. Real-time Updates: Changes to theme settings are applied instantly across the entire application without needing a restart.
5. Robustness: Includes input validation for color hex codes and updated widget tests to ensure the application remains stable.

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PR created automatically by Jules for task 9400552750993925365 started by @Thechopsee

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