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/app/api/auth/register/route.test.ts

@@ -0,0 +1,106 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { prismaMock, resetPrismaMock } from "@/__tests__/helpers/prisma-mock";
+import { createRequest, parseJsonResponse } from "@/__tests__/helpers/api-test-utils";
+import { TEST_USER } from "@/__tests__/helpers/fixtures";
+
+vi.mock("bcryptjs", () => ({
+  default: { hash: vi.fn().mockResolvedValue("$2a$10$hashed") },
+}));
+
+import { POST } from "./route";
+
+beforeEach(() => {
+  resetPrismaMock();
+});
+
+describe("POST /api/auth/register", () => {
+  it("registers a new user successfully", async () => {
+    prismaMock.user.create.mockResolvedValue({
+      ...TEST_USER,
+      id: "new-user",
+      username: "newuser",
+      avatar: "🐱",
+    } as never);
+
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "newuser", password: "password123" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    const { status, data } = await parseJsonResponse(res);
+
+    expect(status).toBe(200);
+    expect(data.username).toBe("newuser");
+    expect(data.id).toBe("new-user");
+    expect(data.avatar).toBe("🐱");
+  });
+
+  it("uses custom avatar if provided", async () => {
+    prismaMock.user.create.mockResolvedValue({
+      ...TEST_USER,
+      avatar: "🦊",
+    } as never);
+
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "newuser", password: "password123", avatar: "🦊" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    const { data } = await parseJsonResponse(res);
+
+    expect(data.avatar).toBe("🦊");
+  });
+
+  it("returns 400 when username is missing", async () => {
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { password: "password123" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 400 when password is missing", async () => {
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "testuser" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 400 when username too short", async () => {
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "a", password: "password123" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 400 when password too short", async () => {
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "testuser", password: "12345" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 409 when username already exists", async () => {
+    const { Prisma } = await import("@prisma/client");
+    prismaMock.user.create.mockRejectedValue(
+      new Prisma.PrismaClientKnownRequestError("Unique constraint", {
+        code: "P2002",
+        clientVersion: "5.0.0",
+      }),
+    );
+
+    const req = createRequest("/api/auth/register", {
+      method: "POST",
+      body: { username: "existing", password: "password123" },
+    });
+    const res = await POST(req, { params: Promise.resolve({}) });
+    expect(res.status).toBe(409);
+  });
+});