feat(PBI-78): cost-analysis UI component (T-903)
- New app/(app)/insights/components/cost-analysis.tsx
- Period selector (7d/30d/90d/MTD) via URL ?period= with useTransition + router.replace
- KPI strip: total cost, avg per day, cache savings, top model
- 2x2 chart grid: daily cost (BarChart), per-model + per-kind (vertical BarCharts), cache efficiency (PieChart)
- Empty state for kpi.jobCount === 0
- Uses MD3 tokens (var(--chart-N), var(--status-done))
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>