feat: add Docker container column in drill-down view

[kostyay]

Summary

• When drilling into a Docker process (com.docker.backend, dockerd, docker-proxy, containerd), an extra Container column appears showing container name, image, and host→container port mapping (e.g., nginx (nginx:latest) 8080→80)
• Uses Docker Engine API via Go SDK with graceful degradation when Docker is unavailable
• New internal/docker package with Resolver interface, async resolution on tick, port-keyed cache
• Container column is sortable via sort mode (s key), with empty values sorting to top

Changes

• internal/docker/ — New package: Resolver interface, dockerResolver impl using Docker Engine API, IsDockerProcess() detection, FormatColumn() display helper
• internal/model/network.go — ContainerInfo, PortMapping types, FormatContainerColumn() with rune-aware truncation
• internal/ui/model.go — SortContainer enum, dockerResolver/dockerCache/dockerView fields
• internal/ui/update.go — Docker detection on drill-down, DockerResolvedMsg handler, async fetchDockerContainers() on tick
• internal/ui/view.go — activeConnectionsColumns() toggles between standard and Docker column sets
• internal/ui/view_table.go — dockerConnectionsColumns() layout, containerColumnValue() port→container lookup
• internal/ui/view_sort.go — SortContainer case with containerSortKey() helper
• go.mod — Added github.com/docker/docker SDK dependency

Test plan

• All existing tests pass (go test ./...)
• Lint clean (make lint), security clean (make security)
• Model type tests (ContainerInfo, PortMapping, FormatContainerColumn with truncation)
• Docker resolver tests with mock API (16 tests)
• UI update tests (drill-down sets dockerView, back clears it, DockerResolvedMsg handling)
• View rendering tests (Container column appears/disappears, matched/unmatched ports)
• Sort tests (ascending, descending, empty values, nil cache)
• Integration tests (full drill-down flow with and without Docker)

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