Show course progress on the "all languages" screen

[SyntaxBlitz]

We can load current course progress on the main screen and render language buttons with this progress. Instead of showing "90 lessons", the button might show "Lesson 12 of 90".

Considerations:

• It's not worth counting the total number of finished lessons; just render the number of the lesson that's "next up".
• This getNextLesson code should be extracted from the LanguageHomeTopButton component so we can reuse it on both screens.