Improve async PI student user experience: step banner, button fix, and prompt update

[sethbern]

This PR fixes some usability issues observed during testing of async peer instruction.

• There is now a step progress banner. Now see a colored banner (Step 1 of 3, 2 of 3, 3 of 3) so they always know where they are in the PI flow. This was confusing before since there is no live instructor
• Previously the "Next Question" button was being pushed far to the right side of the screen, now it stays on the screen regardless of screen size
  • It now says "Done" when on the final question
• Updated prompt to focus on asking students to justify their reasoning instead of adversarially pushing wrong answers

[bnmnetp]

OK, I gotta ask... Have you verified that regular PI continues to function normally with these changes in place?

[sethbern]

Yep, these changes don't touch any sync PI code paths or grading logic, they're scoped to the async template, the async controller function, and the LLM prompt.

I tested sync PI and both text chat and in-person group discussions, and they work as they should.