feat: cron channel targeting + cron management UI page

[HaoweiChan]

Summary

• Add optional channel field to cron jobs, allowing each job to deliver results to a specific connector (e.g. "telegram", "web") instead of always using last-interacted channel. Propagated through CronJob → CronFirePayload → CronListener → ConnectorCenter.
• Add full Cron Jobs UI page (/cron): job listing with status badges, create/edit modal, enable/disable toggle, run-now trigger, delete with confirmation, and recent cron events table.
• Add channel support to cron AI tools (cronAdd, cronUpdate) and API routes.

Changes

Backend

• src/task/cron/engine.ts: channel field on CronJob, CronJobCreate, CronJobPatch, CronFirePayload
• src/task/cron/listener.ts: Pass channel to connectorCenter.notify()
• src/task/cron/tools.ts: channel param on cronAdd and cronUpdate tools
• src/connectors/web/routes/cron.ts: Accept channel in POST body
• src/core/connector-center.ts: channel option in NotifyOpts, targeted delivery in notify() and notifyStream()

Frontend

• ui/src/pages/CronPage.tsx: New page with full CRUD, run-now, events table
• ui/src/App.tsx: Route registration for /cron
• ui/src/components/Sidebar.tsx: Navigation entry with clock icon
• ui/src/api/cron.ts: channel param support in add() and update()
• ui/src/api/types.ts: channel field on CronJob interface

OpenSpec

• openspec/specs/cron-channel-targeting/spec.md
• openspec/specs/cron-ui-page/spec.md

Test plan

• Verify cron jobs page loads at /cron and lists existing jobs
• Create a new cron job with channel targeting (e.g. "telegram")
• Edit an existing job, toggle enable/disable
• Run a job manually and verify output reaches the targeted channel
• Delete a non-heartbeat job
• Verify heartbeat job cannot be deleted
• Verify recent cron events display correctly

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