Add dropdown keyboard accessibility

[eelikivi]

Linked issue: #5609

Problem
The year and month dropdowns are not accessible via keyboard. The dropdown toggles cannot be focused, and the dropdown options cannot be navigated using arrow keys or selected with the Enter key.

Changes

• Changed the dropdown read view components from div to button to allow keyboard focusing.
• Added keyboard support for dropdown option components. Arrow keys can now be used to navigate the list, and the Enter key can be used to select an option.

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.