Safeguard forced práctica unlock with student save grace period and teacher confirmation

Add safeguards around the teacher forced unlock/grade override used when a student edit lock is active. Desired flow: teacher is told that forcing the unlock starts a short grace period rather than immediately overriding the student; student sees a prominent in-app notice that the teacher forced the unlock and has a limited window to save or submit outstanding work; after the window the teacher may continue grading. Product decision still needed: 2 versus 5 minutes, exact eligibility and state transitions, notification behavior for an offline student, and audit events. Preserve the existing emergency override purpose; do not silently discard a student active draft.