feat: 盲盒房间体系重构 — 强制登录、独立房间、用户归属

- 新增 BlindBoxRoom/BlindBoxMember 模型，BlindBoxIdea 增加 userId/drawnById
- 新增房间 API（创建/加入/列表/详情），所有盲盒 API 增加认证和成员校验
- 新建盲盒大厅页面（三层引导式设计：未登录氛围页/首次创建引导/房间列表）
- 新建盲盒房间页面（成员校验/邀请分享/用户归属展示/自动聚焦）
- 首页删除契约画廊和 localStorage 盲盒逻辑，周末契约跳转到 /blindbox
- 清理旧路由 /room/[id]/blindbox
- 提取共享工具 src/lib/blindbox.ts（错误响应/房间号生成/成员校验）
- AuthModal 支持 defaultTab 参数
- 更新项目规范：新项目原则、代码优雅和复用优先

src/lib/blindbox.ts

@@ -0,0 +1,43 @@
+import { prisma } from "@/lib/prisma";
+import { NextResponse } from "next/server";
+
+export function errorResponse(message: string, status: number) {
+  return NextResponse.json({ error: message }, { status });
+}
+
+export function generateRoomCode(): string {
+  const chars = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789";
+  let code = "";
+  for (let i = 0; i < 6; i++) {
+    code += chars[Math.floor(Math.random() * chars.length)];
+  }
+  return code;
+}
+
+export async function generateUniqueRoomCode(): Promise<string> {
+  for (let attempt = 0; attempt < 10; attempt++) {
+    const code = generateRoomCode();
+    const existing = await prisma.blindBoxRoom.findUnique({ where: { code } });
+    if (!existing) return code;
+  }
+  throw new Error("无法生成唯一房间号");
+}
+
+export async function validateMembership(roomId: string, userId: string) {
+  const member = await prisma.blindBoxMember.findUnique({
+    where: { roomId_userId: { roomId, userId } },
+  });
+  return !!member;
+}
+
+export async function getRoomByCode(code: string) {
+  return prisma.blindBoxRoom.findUnique({
+    where: { code },
+    include: {
+      members: {
+        include: { user: { select: { id: true, username: true, avatar: true } } },
+      },
+      _count: { select: { ideas: { where: { status: "in_pool" } } } },
+    },
+  });
+}