UI: Per-agent queue depth and concurrency status

[AbirAbbas]

Ref #316, #319

The backend now exposes GET /api/ui/v1/queue/status which returns per-agent concurrency slot usage. This data isn't surfaced in the UI yet.

What to add:

• Per-agent indicator showing running/max/available execution slots
• Visual feedback when an agent is at capacity (all slots occupied)
• Total running count across all agents

API response shape:

{
  "enabled": true,
  "max_per_agent": 3,
  "total_running": 2,
  "agents": [{
    "agent_node_id": "my-agent",
    "running": 2,
    "max": 3,
    "available": 1
  }]
}

In #316 the user had no way to see that jobs were piling up with no concurrency control.