Onboarding Wizard and Theme Selection

[MaddnRuleZ]

Added The Onboarding Wizard and Dark / White Mode Theme Selection to the Repo

[CLAassistant]

All committers have signed the CLA.

[DennisMoschina]

Thank you for your pull request. Before merging it into the main branch, several issues need to be addressed:

• The logos are not displayed in the dialogue. Please add - lib/widgets/welcome_wizard/assets/ to the assets section in pubspec.yml.
• You mention changing the theme in the app settings, but there are no settings available.
• The onboarding text suggests the main focus of the app is on heart and posture tracking, whereas the primary objective is data collection for research. @TobiasRoeddiger may be able to provide a more appropriate wording.
• The onboarding dialogue appears after every app launch, not just the first.
• There are multiple issues with Flutter check style. Please run flutter analyse and resolve the issues.
• Changing the themes is not supported on Apple platforms. Remove the option to change themes on platforms where it is not supported or implement it in a way that is compatible with all platforms. In the current release, dark mode is already supported on Apple platforms but is applied automatically based on the device’s theme.
• The onboarding wizard uses Material Widgets, which may not be suitable for Apple platforms. For guidance on implementing it in a way that uses more native UI, please refer to https://pub.dev/packages/flutter_platform_widgets/.