Add speed control for letter/number pronunciation

[puppe1990]

Description:

Currently, the speed at which the letters or numbers are spoken is fixed and may not be suitable for all users. It would be beneficial to have a feature that allows the user to adjust the speed at which the letters or numbers are spoken. This would allow users to learn at their own pace and make the app more accessible for users with different learning styles.

Task List:

• Add a speed control option to the interface
• Implement a function that adjusts the playback speed of the audio files based on the user's selection
• Test the feature to ensure it is working as expected

Additional Information:

• The speed control should be added to the settings or options menu.
• The speed control should have a range of options such as slow, medium, and fast.
• The speed control should be remembered between sessions so the user doesn't have to adjust it every time they use the app.