Practice code that survives a refresh

What changed

• Code you write in practice is now saved on your device as you type, for each problem and language. It comes back after a refresh, a closed tab, or signing in, and it clears once you solve the problem.
• When a submit has failing tests, the results stay open so you can read them. Going to the next problem and back now returns you to the one you were on, and problem pages no longer fill the Examples section with generated copies of the first example.
• The Lab opens with a compact progress row, so the problem list starts higher on the page, and the full panels are one click away. Rena's panel on a problem page can now be dragged open to full width.