Use Inertia\Support\Header constants instead of strings

[onlime]

Using Inertia's Header constants and introducing Momentum Modal ModalHeader constants.

[what-the-diff]

PR Summary

• Improved Test Configuration
  • The testing configuration has been updated to a newer version. Additionally, the settings for what code parts are covered by testing have been simplified, now covering all the main source code.
• Refined User Interface Class
  • The class handling the user interface has been tweaked for better clarity and easy maintenance. It now uses constants instead of plain text, ensuring consistency and reducing chances of error.
• Introduced Modal Header Class
  • A new class has been designed to carry key definitions for interfaces. This provides a single source of truth for these definitions and allows for better organization.
• Improved Test File Consistency
  • How headers are referred to in the test file has been standardized, using constant definitions from the aforementioned classes. This promotes consistency in the code.