feat: web UI dashboard for monitoring and management

[STRRL]

Problem

Currently the only way to observe the controller is by reading log files. There's no visual overview of runner status, job activity, or system health.

Proposal

Add a built-in web UI dashboard served by the controller process (e.g. :8080).

Views

Overview

• Runner sets with current status (idle/busy/preparing count per set)
• Live job queue depth
• System health indicators (CPU, memory, disk)

Runners

• List of all active runners with state, uptime, current job
• Historical runners with job result and duration

Jobs

• Job execution timeline (Gantt-style)
• Job history table with filtering (by runner set, result, date range)
• Duration breakdown: queue wait → preparation → execution

Resources (relates to #17)

• CPU/memory usage charts over time
• Per-runner-set resource consumption
• Disk and network I/O

Configuration

• Current config (read-only, secrets redacted)
• Runner set labels and limits
• Image versions in use

Logs (relates to #15, #16)

• Live log tail from controller
• Per-job log viewer

Technical Considerations

• Embedded web server in the controller binary (no separate process)
• Static assets embedded via embed.FS
• Lightweight frontend (plain HTML + HTMX, or minimal React)
• WebSocket for live updates (runner state changes, log streaming)
• Optional: disable via --no-dashboard flag
• Auth: optional basic auth or token for remote access

Related

• chore: improve controller logging format and verbosity #15 (controller logging)
• feat: collect and forward runner job logs #16 (runner job logs)
• feat: track job execution history and resource consumption metrics #17 (job history and resource metrics)