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/tests/dashboard/dashboard-cards-registry.test.ts

@@ -0,0 +1,36 @@
+import { afterEach, describe, expect, it } from "vitest";
+import {
+  registerDashboardCard,
+  registeredDashboardCardList,
+  unregisterDashboardCard,
+} from "@/dashboard/dashboard-cards-registry";
+
+const FAKE_CARD_ID = "__test_plugin_dashboard_card__";
+
+describe("registerDashboardCard seam", () => {
+  afterEach(() => {
+    unregisterDashboardCard(FAKE_CARD_ID);
+  });
+
+  it("is not registered by default", () => {
+    expect(registeredDashboardCardList()).toEqual([]);
+  });
+
+  it("registers and unregisters a plugin dashboard card", () => {
+    registerDashboardCard({
+      id: FAKE_CARD_ID,
+      titleKey: "nav.fakePluginItem",
+      render: () => null,
+    });
+
+    expect(registeredDashboardCardList().map((c) => c.id)).toContain(
+      FAKE_CARD_ID,
+    );
+
+    unregisterDashboardCard(FAKE_CARD_ID);
+
+    expect(registeredDashboardCardList().map((c) => c.id)).not.toContain(
+      FAKE_CARD_ID,
+    );
+  });
+});