Whiteboard grading and copilot suggestions

What changed

• Correct whiteboard drawings in live classroom games now earn the streak bonus, the same as other correct answers.
• AI grading for whiteboard drawings now works on questions without a reference image and is included with paid Classroom plans. Hosts can still grade drawings by hand, and the leaderboard marks scores still waiting on a grade.
• Educator Copilot and Student Copilot now open with suggestions for their own audience and can answer questions about how AlgoArena works. Student Copilot now points learners to problems in the Lab they can open and solve, instead of writing new ones.