test: 添加完整测试套件（52 个文件，326 个用例）

基于 Vitest 搭建测试基础设施，覆盖后端纯函数、API 路由、
前端 hooks、UI 组件和页面级集成测试。

src/__tests__/helpers/prisma-mock.ts

@@ -0,0 +1,13 @@
+import { vi } from "vitest";
+import { mockDeep, mockReset, type DeepMockProxy } from "vitest-mock-extended";
+import type { PrismaClient } from "@prisma/client";
+
+export const prismaMock = mockDeep<PrismaClient>();
+
+vi.mock("@/lib/prisma", () => ({
+  prisma: prismaMock,
+}));
+
+export function resetPrismaMock() {
+  mockReset(prismaMock);
+}