Whiteboard lessons and bigger Learn practice sets

What changed

• Readings in the vibecoding course are now taught by Rena on a whiteboard, one step at a time, and she reads them aloud in your browser's voice.
• Signed-in learners can stop her at any point to ask about what is on the board. Her closing question appears once the lesson reaches its end.
• Every lesson in the 17 Learn language tracks now has about 20 practice problems, from filling in one gap to starting from an empty editor. Starter code for loop exercises no longer includes the finished answer.