Build a System section and Rena in course labs

What changed

• Systems practice now has its own Build a System section in the Lab, with search plus difficulty and company filters. Browsing is free, and opening a question needs Pro.
• Library rows now list the most telling company matches first, so large companies are no longer crowded out by companies whose interviews cover a bit of everything. Ten more companies joined the company filter, matched by the difficulty of what they ask.
• Rena now sits beside every course lab and gives its feedback, and in Builder labs she takes you through the steps one at a time. Practical Vibecoding progress now comes from finished work, so a quiz counts once you pass it and a reading counts once you answer Rena's closing question correctly.