feat(ui): dashboard — FeeChart with time window toggle

[Tinna23]

Overview

Build the FeeChart component — a Recharts bar chart showing fee history over selectable time windows, with spike annotations.

Acceptance Criteria

• Component at src/app/components/dashboard/FeeChart.tsx
• Uses Recharts (BarChart, Bar, XAxis, YAxis, Tooltip, ResponsiveContainer)
• Time window toggle: 1H | 6H | 24H — buttons above the chart
• On window change, fetches fetchFeeHistory(window) from lib/api.ts and re-renders
• X-axis: timestamps formatted to readable time labels
• Y-axis: fee values in stroops
• Bar colour: primary green (#00ff9d); spike bars coloured red/yellow
• Spike detection: bars where max_fee > avg_fee * 3 rendered in accent colour with a ReferenceLine or custom dot annotation
• Custom Tooltip showing: time, base fee, avg fee, max fee
• Accepts props: { initialWindow?: '1h' | '6h' | '24h' }
• Responsive — fills container width via ResponsiveContainer
• Loading state while fetching history

Notes

• FeeChart manages its own history fetch state independently (separate from DashboardShell polling)
• Install Recharts: npm install recharts