feat(analytics): add 6-week trend chart, daily activity, and richer admin views

- SVG line+area chart for weekly attempts vs completions (6 weeks)
- 30-day daily activity bar strip
- Score distribution bars by 20% buckets
- Top students table with avg best, sets attempted, last-7d activity
- Per-set engagement table with attempts, students, avg best, ≥80% rate
- Two new metric cards: active students last 7d, attempts last 7d
- Two-column row layout and consistent empty states for tighter spacing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: 2 people