Assessment history now uses the shared browse-sidebar pattern

Assessment history now follows the same navigation pattern used across the platform.

• Recent assessments are presented in a compact left sidebar that matches the browse sidebar structure.
• The active assessment is easier to identify through the shared active-state styling.
• Assessment mode no longer needs a separate visual language for history navigation.

This makes it easier for learners to move between previous assessment results while keeping the assessment surface aligned with the rest of Upsoma.