feat: Cron management UI (GH #253)

[vivekchand]

Summary

Full cron job management UI in the ClawMetry dashboard. Closes #253.

What changed

Cron list with full CRUD

• Run Now button fires the job on demand (confirmation dialog already present)
• Enable/Disable toggle per job with instant optimistic UI update
• Edit button opens the existing edit modal to modify schedule, prompt, channel, model
• Delete button with confirmation dialog
• New Job button to create cron jobs without leaving the dashboard
• All buttons were previously commented out; this PR enables them now that gateway CRUD has been validated

Auto-refresh

• Polls /api/crons every 30s automatically when the Crons tab is active
• Checkbox toggle to pause/resume auto-refresh
• Timer is cleaned up when switching to another tab (no background polling)

Human-readable schedule descriptions

• Cron expressions now show a plain-English label alongside the raw expression
• Examples: '*/30 * * * *' shows 'every 30 minutes', '0 9 * * 1-5' shows 'weekdays at 09:00', '0 * * * *' shows 'every hour at :00'
• 'every Xmin' intervals show 'every Xh' when >= 60 minutes

Multi-node cron status panel

• Appears above the job list when fleet nodes are registered via /api/nodes
• Per-node card shows: online/offline indicator, node name, job count, error count, last run time
• Hidden automatically when no nodes are in the fleet
• Pulls cron summary from node latest_metrics.crons field

Execution history

• Already implemented (expandable rows with calendar heatmap, run log modal)
• No changes needed; works with the existing /api/cron/{id}/runs endpoint

Testing

• Python syntax validated: python3 -m py_compile dashboard.py
• No regressions: all existing cron JS functions (cronRunNow, cronToggle, cronEdit, cronDelete, cronCreateNew, loadCronRuns, loadCronLog) are unchanged in logic; only the commented-out lines are uncommented