Create CONTRIBUTING.md for project guidelines

Added a comprehensive contributing guide for the ChatCrystal project, including sections on development setup, workflow, commit conventions, and bug reporting.

Author: ZengLiangYi

CONTRIBUTING.md

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+# Contributing to ChatCrystal
+
+感谢你对 ChatCrystal 的关注！我们欢迎任何形式的贡献。
+Thanks for your interest in ChatCrystal! We welcome contributions of all kinds.
+
+---
+
+## 目录 / Table of Contents
+
+- [开始之前 / Before You Start](#开始之前--before-you-start)
+- [开发环境 / Development Setup](#开发环境--development-setup)
+- [项目结构 / Project Structure](#项目结构--project-structure)
+- [开发流程 / Development Workflow](#开发流程--development-workflow)
+- [提交规范 / Commit Convention](#提交规范--commit-convention)
+- [Pull Request 流程 / Pull Request Process](#pull-request-流程--pull-request-process)
+- [报告 Bug / Reporting Bugs](#报告-bug--reporting-bugs)
+- [功能建议 / Feature Requests](#功能建议--feature-requests)
+
+---
+
+## 开始之前 / Before You Start
+
+- 请先阅读 [Code of Conduct](CODE_OF_CONDUCT.md) / Please read our [Code of Conduct](CODE_OF_CONDUCT.md)
+- 对于重大变更，请先开 Issue 讨论 / For major changes, please open an Issue first to discuss
+- 确保你的 Node.js 版本 >= 20 / Make sure you have Node.js >= 20
+
+## 开发环境 / Development Setup
+
+```bash
+# 1. Fork 并克隆仓库 / Fork and clone the repo
+git clone https://github.com/<your-username>/ChatCrystal.git
+cd ChatCrystal
+
+# 2. 安装依赖 / Install dependencies
+npm install
+
+# 3. 启动开发服务器（前端 + 后端）/ Start dev server (frontend + backend)
+npm run dev
+```
+
+启动后：
+- 后端 API: `http://localhost:3721`
+- 前端页面: `http://localhost:13721`
+
+After starting:
+- Backend API: `http://localhost:3721`
+- Frontend: `http://localhost:13721`
+
+### 代码检查 / Linting
+
+```bash
+npm run lint        # 检查 / Check
+npm run lint:fix    # 自动修复 / Auto-fix
+```
+
+## 项目结构 / Project Structure
+
+```
+ChatCrystal/
+├── shared/     # 共享类型 (无构建步骤) / Shared types (no build step)
+├── server/     # Fastify 后端 / Fastify backend
+├── client/     # React 前端 (Vite + Tailwind CSS) / React frontend
+├── electron/   # Electron 桌面应用 / Electron desktop app
+└── scripts/    # 发布与工具脚本 / Release & utility scripts
+```
+
+## 开发流程 / Development Workflow
+
+1. 从 `main` 创建你的分支 / Create a branch from `main`
+   ```bash
+   git checkout -b feat/your-feature
+   ```
+2. 进行修改并确保 lint 通过 / Make changes and ensure lint passes
+3. 提交你的更改 / Commit your changes（见下方提交规范 / see commit convention below）
+4. 推送到你的 Fork / Push to your fork
+5. 创建 Pull Request / Open a Pull Request
+
+## 提交规范 / Commit Convention
+
+使用 [Conventional Commits](https://www.conventionalcommits.org/) 格式：
+Use [Conventional Commits](https://www.conventionalcommits.org/) format:
+
+```
+<type>(<scope>): <description>
+```
+
+常用 type / Common types:
+| type | 说明 / Description |
+|------|-------------------|
+| `feat` | 新功能 / New feature |
+| `fix` | Bug 修复 / Bug fix |
+| `docs` | 文档变更 / Documentation |
+| `refactor` | 重构（不改变行为）/ Refactoring (no behavior change) |
+| `style` | 代码格式 / Code style (formatting, etc.) |
+| `test` | 测试 / Tests |
+| `chore` | 构建/工具变更 / Build or tooling changes |
+
+常用 scope / Common scopes:
+`client`, `server`, `shared`, `electron`, `cli`, `docs`, `ci`
+
+示例 / Examples:
+```
+feat(client): add dark mode toggle
+fix(server): handle empty conversation parsing
+docs: update README installation steps
+```
+
+## Pull Request 流程 / Pull Request Process
+
+1. 确保 PR 描述清晰，说明做了什么以及为什么 / Ensure PR description explains what and why
+2. 关联相关 Issue（如有）/ Link related Issues (if any)
+3. 确保 CI 检查通过 / Ensure CI checks pass
+4. 等待维护者审核 / Wait for maintainer review
+5. 根据反馈进行修改 / Address review feedback
+
+### PR 标题格式 / PR Title Format
+
+与提交规范一致 / Same as commit convention:
+```
+feat(client): add dark mode toggle
+```
+
+## 报告 Bug / Reporting Bugs
+
+使用 [Bug Report](https://github.com/ZengLiangYi/ChatCrystal/issues/new?template=bug_report.yml) 模板，包含：
+Use the [Bug Report](https://github.com/ZengLiangYi/ChatCrystal/issues/new?template=bug_report.yml) template, including:
+
+- 清晰的复现步骤 / Clear steps to reproduce
+- 期望与实际行为 / Expected vs actual behavior
+- 版本与运行环境 / Version and environment
+- 相关日志或截图 / Relevant logs or screenshots
+
+## 功能建议 / Feature Requests
+
+使用 [Feature Request](https://github.com/ZengLiangYi/ChatCrystal/issues/new?template=feature_request.yml) 模板。
+Use the [Feature Request](https://github.com/ZengLiangYi/ChatCrystal/issues/new?template=feature_request.yml) template.
+
+---
+
+如有疑问，请在 [Discussions](https://github.com/ZengLiangYi/ChatCrystal/discussions) 中提问。
+If you have questions, feel free to ask in [Discussions](https://github.com/ZengLiangYi/ChatCrystal/discussions).