chore: 添加 Cursor 设计系统和项目规范规则

- design-system.mdc: 暗色 token 映射表、禁止白色背景、按钮分层规则
- project-conventions.mdc: 技术栈声明、Tailwind v4 语法、组件和 API 编码规范

.cursor/rules/project-conventions.mdc

@@ -0,0 +1,34 @@
+---
+description: Project-wide coding conventions for NoWhatever
+alwaysApply: true
+---
+
+# NoWhatever Project Conventions
+
+## Tech Stack
+
+- **Framework**: Next.js App Router (all pages are `"use client"`)
+- **Database**: Prisma + SQLite
+- **Styling**: Tailwind CSS v4 (use `bg-linear-to-*` NOT `bg-gradient-to-*`)
+- **Animation**: framer-motion
+- **Icons**: lucide-react
+- **Effects**: canvas-confetti
+
+## Tailwind v4 Syntax
+
+- Gradients: `bg-linear-to-br` (not `bg-gradient-to-br`)
+- Arbitrary values: prefer Tailwind built-in classes over `[]` when possible (e.g. `max-w-20` not `max-w-[5rem]`)
+
+## Component Patterns
+
+- Page components export `default function PageName()`
+- Use `useCallback` for event handlers passed as props
+- 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
+
+## API Routes
+
+- Located in `src/app/api/`
+- Return `NextResponse.json()` with appropriate status codes
+- Always handle errors with try/catch