feat(portal-shell): extract domain API layer and migrate 31 widgets

Task 4-10 of portal-shell data abstraction plan (M1-M2).

Add 7 domain API modules under src/lib/api/ (parent/admin/teacher/
student/universal/sidebar/topbar), each exposing semantic hooks that
wrap useWidgetQuery/useWidgetMutation and return flattened domain
models. Widget code now imports from @/lib/api instead of inlining
gql literals.

- 31 widgets migrated (gql literal count in widgets: 0)
- 7 test files (85 cases, all passing)
- topbar.useNotifications renamed to useNotificationBell to avoid
  barrel export collision with universal.useNotifications
- typecheck + lint (0 errors) + test (85/85) verified

apps/portal-shell/src/widgets/topbar/locale-switcher/plugin.manifest.ts

@@ -0,0 +1,20 @@
+/**
+ * locale-switcher 插件清单（topbar）
+ *
+ * 语言切换器，插入 top slot。通过 usePluginStore 读写 locale。
+ */
+import type { PluginManifest } from "@/lib/types";
+
+export const manifestMeta: Omit<PluginManifest, "Component"> = {
+  pluginId: "locale-switcher",
+  version: "0.1.0",
+  requiredShellVersion: "^1.0.0",
+  metadata: {
+    displayName: "语言切换",
+    description: "切换界面语言（zh-CN / en）",
+    category: "topbar",
+    requiredRoles: ["teacher", "student", "parent", "admin"],
+    defaultSlot: "top",
+    defaultSize: { colSpan: 1, rowSpan: 1 },
+  },
+};