[Observability] Distributed traces: gateway endpoint and dashboard panel

### Feature Summary
Distributed traces cannot be retrieved or visualized in the dashboard. This adds trace retrieval through the gateway plus a trace-tree UI panel.
### Proposed Solution or Design
- Backend: a trace query builder and `parseTraces`; a `TracesResource` exposing trace retrieval by id, with id validation and scope enforcement at the gateway.
- Frontend: a traces panel rendering spans as a tree built from parent-span relationships, with pivot-from-log so a log row can open its trace directly. Span fields are bound with text interpolation only.
### Affected Area
Workflow UI, Other
### Related
Part of #4070. Depends on #5372.