feat: ship docker as a first-party in-process plugin

Moves docker backend and frontend into plugins/docker/, alongside
ssh-terminal. Adds a host editor tab seam, a dashboard card registry,
and a build step to compile plugin backends into dist.

src/ui/shell/tabUtils.tsx

@@ -85,7 +85,7 @@ const loadFileManager = () =>
   }));
 const FileManager = lazy(loadFileManager);
 const loadDockerManager = () =>
-  import("@/features/docker/DockerManager").then((m) => ({
+  import("../../../plugins/docker/frontend/DockerManager").then((m) => ({
     default: m.DockerManager,
   }));
 const DockerManager = lazy(loadDockerManager);