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.

plugins/docker/frontend/index.mjs

@@ -0,0 +1,68 @@
+/**
+ * Frontend half of the docker plugin.
+ *
+ * Registers the container-management tab through the existing extension
+ * seams (registerRailItem / registerTabComponent) the same way
+ * ssh-terminal's frontend does, and the host-editor Docker tab through the
+ * new registerHostEditorTab seam built for this plugin.
+ *
+ * HostDockerTab itself stays in src/ui/sidebar/HostEditorFeatureTabs.tsx
+ * rather than moving into this plugin: it takes the same {form, setField}
+ * pair every other host editor tab does, and those types (HostEditorForm,
+ * the setField signature) are core types HostEditor.tsx owns, the same
+ * reason ssh-terminal leaves session-manager.ts in core rather than copying
+ * it here.
+ *
+ * register() is called when the plugin is enabled, unregister() when it is
+ * disabled. As of this change neither is actually invoked anywhere yet --
+ * see plugins/ssh-terminal/frontend/index.mjs and
+ * src/ui/tests/sidebar/plugin-extension-seam.test.tsx, which says outright
+ * "there is no plugin loader yet" for the frontend half. This file follows
+ * the same shape so it is ready the moment that loader exists.
+ */
+
+export const id = "docker";
+export const tabId = "docker";
+export const hostEditorTabId = "docker";
+
+export async function register({
+  registerRailItem,
+  registerTabComponent,
+  registerHostEditorTab,
+  icons,
+}) {
+  registerTabComponent(tabId, () =>
+    import("./DockerManager.tsx").then((m) => ({
+      default: m.DockerManager,
+    })),
+  );
+
+  registerRailItem({
+    id: tabId,
+    icon: icons.Box,
+    labelKey: "nav.docker",
+    kind: "tab",
+  });
+
+  if (registerHostEditorTab) {
+    const { HostDockerTab } = await import(
+      "../../../src/ui/sidebar/HostEditorFeatureTabs.tsx"
+    );
+    registerHostEditorTab({
+      id: hostEditorTabId,
+      labelKey: "hosts.tabDocker",
+      icon: icons.Box,
+      component: HostDockerTab,
+    });
+  }
+}
+
+export async function unregister({
+  unregisterRailItem,
+  unregisterTabComponent,
+  unregisterHostEditorTab,
+}) {
+  unregisterRailItem(tabId);
+  unregisterTabComponent(tabId);
+  unregisterHostEditorTab?.(hostEditorTabId);
+}