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/lib/api/index.ts

@@ -2,9 +2,16 @@
  * API 层统一出口
  *
  * widget 通过 `import { useParentChildren } from "@/lib/api"` 调用。
- * 各 domain 文件在 M2 阶段逐步加入。
+ * 7 个 domain 文件覆盖全部 31 widget。
  *
  * 关联：spec §2.2
  */
 export * from "./errors";
 export * from "./types";
+export * from "./universal";
+export * from "./sidebar";
+export * from "./topbar";
+export * from "./teacher";
+export * from "./student";
+export * from "./parent";
+export * from "./admin";