Add evaluation score trend sparkline to dashboard overview #12
Description
Show a small sparkline chart next to the average score on the dashboard overview page, showing score trend over the last 7 days.
Acceptance criteria:
- SVG sparkline (no chart library needed)
- Shows last 7 days of average eval scores
- Color: green if trending up, red if trending down
- Fits inline next to the score number
Good starting point: dashboard/src/pages/Dashboard.tsx