feat: add gamified, level-aware practice mode for students

[Ritwij07]

Backend:

• New GET /api/students/:id/practice endpoint. Reuses the existing generateQuestionsForLevel() level generator (the same one that powers real worksheet PDFs) so practice questions always match the student's real currentLevel/currentSubLevel — never a generic or hardcoded set.
• Skips the Puppeteer/PDF pipeline entirely for speed, since this is a fast in-class activity, not an official exam paper.
• Optional ?topic= query param to target one weak area at a time.
• Deliberately does NOT mutate the official student.streak or levelHistory fields used by the assessment-cycle/evaluation pipeline — practice-session state stays session-local so this can't interfere with the official 3x/year assessment cycle logic other in-flight PRs are working on.
• Uses the same getAuthUser/canAccessStudent auth pattern as the existing diagnostic endpoint.

Frontend:

• New PracticeMode.tsx: an interactive practice session — one question at a time, immediate correct/incorrect feedback, a live streak counter, a progress bar, and an end-of-session summary. Supports the existing number/text/choice question types.
• Wired into PanelViews.tsx's 'Recommended Focus Areas' panel, which previously only showed static text ('Additional practice recommended for X') with no action attached to it. Added a 'Start Practice' button plus a per-topic 'Practice this ->' button so a teacher can launch a session targeted at a specific weak area directly from the student profile.

This turns currentLevel/currentSubLevel/streak — fields that already existed in the data model and were only ever displayed as static numbers — into an actual interactive learning activity, which the project's own README describes as the goal ('gradual, fun activities') but nothing in the codebase implemented yet.