feat(frontend): host les fenêtres plugin et l'API publique windows.open

Ajoute la vue window host qui charge le layout plugin dans une fenêtre OS
détachée, expose react/react-dom/jsx-runtime hébergés sous
frontend/public/plugin-host pour que le runtime SDK y résolve React, et
câble services.windows.open() côté runtime plugin (loader/services/registry).

npm test -- --run src/adapters/window.test.ts src/app/ViewWindow.test.tsx
  src/plugins/runtime/loader.test.ts src/plugins/runtime/services.test.ts : 39/39
npm run typecheck : OK
npm run build : OK

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/adapters/window.test.ts

@@ -29,6 +29,41 @@ describe("TauriWindowGateway (#23)", () => {
     });
   });
 
+  it("opens plugin layout windows through the backend command", async () => {
+    invoke.mockResolvedValueOnce({
+      label: "view-plugin-layout",
+      url: "index.html?pluginLayout=1",
+      alreadyOpen: false,
+      providerPluginDisplayName: "Plugin",
+      layoutLabel: "Dashboard",
+      surface: {
+        pluginId: "dev.acme.plugin",
+        layoutType: "dev.acme.plugin.dashboard",
+        state: { tab: "overview" },
+      },
+    });
+    const gw = new TauriWindowGateway();
+
+    await expect(
+      gw.openPluginLayoutWindow({
+        pluginId: "dev.acme.plugin",
+        layoutType: "dev.acme.plugin.dashboard",
+        state: { tab: "overview" },
+      }),
+    ).resolves.toMatchObject({
+      label: "view-plugin-layout",
+      surface: { layoutType: "dev.acme.plugin.dashboard" },
+    });
+
+    expect(invoke).toHaveBeenCalledWith("open_plugin_layout_window", {
+      input: {
+        pluginId: "dev.acme.plugin",
+        layoutType: "dev.acme.plugin.dashboard",
+        state: { tab: "overview" },
+      },
+    });
+  });
+
   it("onViewWindowClosed listens on the lifecycle channel and forwards only closes", async () => {
     // Capture the raw Tauri listener so we can drive lifecycle events at will.
     let raw: ((e: { payload: unknown }) => void) | undefined;