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

.cursor/rules/project-conventions.mdc

@@ -5,6 +5,13 @@ alwaysApply: true
 
 # NoWhatever Project Conventions
 
+## Philosophy
+
+- This is a **new project** — never worry about backward compatibility or legacy code migration
+- Prioritize **code elegance and reuse** over quick hacks
+- Extract shared logic into reusable utilities (`src/lib/`) and components (`src/components/`)
+- DRY: if the same pattern appears twice, extract it
+
 ## Tech Stack
 
 - **Framework**: Next.js App Router (all pages are `"use client"`)
@@ -26,9 +33,12 @@ alwaysApply: true
 - Use `motion.*` from framer-motion for animated elements
 - Mobile-first, `min-h-dvh` for full-height pages
 - `overflow-y-auto scrollbar-none` for scrollable pages
+- Extract reusable UI patterns into shared components (modals, cards, empty states)
 
 ## API Routes
 
 - Located in `src/app/api/`
 - Return `NextResponse.json()` with appropriate status codes
 - Always handle errors with try/catch
+- Extract common validation (userId, membership) into utility functions in `src/lib/`
+- Use consistent error response shape: `{ error: string }`