Fix brightness detection and integrate dynamic color support

[adeeteya]

This pull request introduces dynamic color support to the application, allowing it to use system-provided color schemes when available, and falls back to a default color if not. The changes also include the necessary setup for the dynamic_color package across all platforms and update the theme logic in the main app.

Dynamic color support and platform integration:

• Added the dynamic_color package to pubspec.yaml and imported it in main.dart to enable dynamic color theming. [1] [2]
• Updated theme logic in MarkdownEditorApp to use dynamic color schemes (DynamicColorBuilder) for both light and dark themes, falling back to Colors.indigo when dynamic colors are unavailable. [1] [2]

Platform-specific plugin registration:

• Registered the dynamic_color plugin for Linux, macOS, and Windows by updating the respective generated plugin registrant files and CMake lists. [1] [2] [3] [4] [5]

Minor code improvements:

• Replaced PlatformDispatcher.instance with WidgetsBinding.instance.platformDispatcher for consistency and to avoid deprecation warnings in device_preference_notifier.dart. [1] [2]

Fixes #35