[Bug]: Create a Dummy Donate Page with Blood Group Selection (home_screen.dart)

The "Donate" button on the home page currently lacks navigational logic and functionality. Implement a dummy DonatePage that includes a basic blood group selection feature to simulate the donation process. This page will act as a placeholder until further development is added.

### Expected behavior

Adding this feature will give users a more interactive experience and set the foundation for future enhancements. It makes it clear that the donation functionality is planned and allows users to select their blood group for a more realistic interface.

Create a new DonatePage widget as a placeholder.
Add a DropdownButton to the DonatePage for selecting blood groups (e.g., A+, A-, B+, B-, AB+, AB-, O+, O-).
Update the onPressed callback of the "Donate" button on the home page to navigate to this new DonatePage.
Include basic UI elements such as a title and a submit button.

Start in home_screen.dart and trace the existing Donate button's onPressed callback. Create the requested DonatePage with a title, blood-group DropdownButton, and submit button, then connect the button to navigation and run the app to verify the page opens and a blood group can be selected.

Written by the indexing model from the issue text.