feat(web): add per-container log filtering to the log pane

[breezy-devs]

Closes #3814

Summary

• Adds a container structured field to every pod log line in the streaming controller, enabling frontend filtering without text parsing
• Adds a container filter dropdown to the log pane action bar, shown when a resource has logs from 1+ distinct containers: disabled at exactly 1 (layout stays stable once any container logs arrive), interactive at 2+
• Resources with no container logs (e.g. local_resource) never render the dropdown
• Filter selection is URL-persisted via the existing search param pattern and integrates with the existing FilterSet pipeline

Multi-Container

Single Container

No container

Test plan

• Run go test ./internal/controllers/core/podlogstream/... to verify the new TestLogContainerField test passes
• Run yarn test in web/ to verify all frontend tests pass
• Deploy a multi-container resource (e.g. app + sidecar) and confirm the container dropdown appears and is interactive
• Verify selecting a container filters logs correctly and the selection round-trips through the URL
• Verify a single-container resource shows the dropdown disabled with the container name in the right pill
• Verify a resource with no container logs (e.g. local_resource) shows no dropdown