feat(portal-shell): implement portal-shell with apollo-router integration

M8: portal-shell unified frontend shell (Modular Monolith + micro-kernel).

- Apollo Client -> apollo-router (port 4010, RSC prefetch)

- 5 layouts: classic/focus/split/triple/canvas

- Registry + PluginLoader (dynamic import ssr:false)

- 3-layer props merge, Zustand PluginStore

- 4 widgets: grades/notification-bell/user-menu/class-selector

- config-service: new pluginConfig GraphQL resolver

- apollo-router: CORS + header propagation for portal-shell

- docker-compose.yml: portal-shell service block

apps/portal-shell/src/app/page.tsx

@@ -0,0 +1,8 @@
+import { redirect } from "next/navigation";
+
+/**
+ * 根路径重定向到 /shell（portal-shell spec §8.2：路由前缀 /shell/*）。
+ */
+export default function RootPage(): never {
+  redirect("/shell");
+}