New /exchange page for Bitcoin trading:
- Buy/Sell direction toggle with visual feedback
- Amount slider (€100-€3000 in €20 increments)
- Live price display with auto-refresh every 60s
- Direction-specific pricing (buy +5%, sell -5%)
- Sats amount displayed in BTC format (0.XX XXX XXX)
- Date selection with availability indicators
- Time slot selection grid
- Confirmation panel with trade summary
Regenerate frontend types from updated OpenAPI schema.