feat: show pool reason on dashboard and droplet detail

[MichielDean]

Summary

• Pooled droplets in the dashboard are now clickable — clicking expands a row to show the pool reason inline
• Droplet detail page now shows a "Pool Reason" section (red-bordered callout) for pooled droplets
• Adds GetPoolReasons() and GetPoolReason() methods to the cistern client, querying the events table for pool event payloads
• DashboardData now includes pool_reasons: Record<string, string> mapping pooled droplet IDs to their reasons
• PooledSection passes poolReasons in, DropletRow gains onClick and expanded props for interactive expansion
• Droplet IDs in DropletRow are now clickable links to the detail page (navigate via react-router-dom)
• Single-droplet API (GET /api/droplets/{id}) now includes pool_reason field for pooled droplets
• All existing tests updated for new pool_reasons field

Changed files

• internal/cistern/client.go — GetPoolReasons(), GetPoolReason() methods
• cmd/ct/dashboard.go — PoolReasons in DashboardData, populated in fetchDashboardData
• cmd/ct/dashboard_web.go — handleGetDroplet injects pool_reason for pooled droplets
• web/src/api/types.ts — pool_reasons on DashboardData, pool_reason on Droplet
• web/src/pages/Dashboard.tsx — PooledSection with expandable rows showing reasons
• web/src/pages/DropletDetail.tsx — Pool Reason section for pooled droplets
• web/src/components/DropletRow.tsx — Clickable ID links, onClick/expanded props
• web/src/__tests__/ — Updated mock data with pool_reasons