UI: Show error_category on failed executions

[AbirAbbas]

Ref #316, #319

Execution failures now include an error_category field that classifies the root cause. The UI should display this prominently on failed executions.

Categories to handle:

Category	Meaning	Suggested UX
llm_unavailable	LLM backend circuit breaker is open	Red badge, link to LLM health
concurrency_limit	Agent at max concurrent executions	Yellow badge, link to queue status
agent_timeout	Agent didn't respond in time	Orange badge
agent_unreachable	Agent is down	Red badge, link to node status
agent_error	Agent returned an error	Red badge with error details
bad_response	Agent returned invalid response	Red badge
internal_error	Control plane error	Red badge

What to add:

• Error category badge/tag on execution list and detail views
• Human-readable description of what went wrong
• Where applicable, link to the relevant diagnostic page (LLM health, queue, node status)

The user in #316 had no way to tell if the problem was the LLM, the agent, or the control plane.