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/sidebar/term-switcher/index.tsx

@@ -0,0 +1,57 @@
+"use client";
+
+/**
+ * term-switcher（sidebar / side）
+ *
+ * 通过 useTerms 查询 apollo-router → core-edu 子图的 terms 数据。
+ * 切换学期时写入 URL Search Params（termId），其他插件自动响应。
+ * 默认选中 isActive=true 的学期。
+ *
+ * 关联：portal-shell spec §5.2.1 URL 驱动、M8 验收
+ */
+import { useRouter, useSearchParams } from "next/navigation";
+import { useTerms } from "@/lib/api/sidebar";
+import { PluginSkeleton } from "@/shell/PluginLoader";
+import type { PluginProps } from "@/lib/types";
+
+export default function TermSwitcher(_props: PluginProps): React.ReactElement {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const currentTermId = searchParams.get("termId") ?? "";
+
+  const { data, loading } = useTerms();
+
+  const handleSelect = (termId: string): void => {
+    const params = new URLSearchParams(searchParams.toString());
+    params.set("termId", termId);
+    router.push(`?${params.toString()}`);
+  };
+
+  if (loading && !data) {
+    return <PluginSkeleton variant="list" />;
+  }
+
+  const terms = data ?? [];
+  const activeTerm = terms.find((t) => t.isActive);
+  // URL 未指定 termId 时默认选中当前激活学期
+  const selectedTermId = currentTermId || activeTerm?.id || "";
+
+  return (
+    <div>
+      <p className="text-small text-ink-muted">学期</p>
+      <select
+        value={selectedTermId}
+        onChange={(e) => handleSelect(e.target.value)}
+        className="mt-xs w-full rounded-button border border-rule bg-surface px-sm py-xs text-small text-ink"
+        aria-label="选择学期"
+      >
+        <option value="">请选择学期</option>
+        {terms.map((t) => (
+          <option key={t.id} value={t.id}>
+            {t.name}
+          </option>
+        ))}
+      </select>
+    </div>
+  );
+}