refactor: 重组项目结构，前端收拢至 frontend/，新增 backend/ 目录

- 前端文件移入 frontend/ 子目录
- 新建 backend/ 目录（待开发）
- 新增 CLAUDE.md、claudehd.md、EXTERNAL_SERVICES.md

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,53 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Overview
+
+This is a 1:1 pixel-perfect clone of the Google Gemini chat interface, built with Next.js 16 (App Router), React 19, TypeScript, Tailwind CSS 4, and shadcn/ui. It was generated via [v0.app](https://v0.app/chat/lx6pc6oofZh) and auto-synced to this repo. Deployed on Vercel.
+
+## Commands
+
+```bash
+npm install        # Install dependencies
+npm run dev        # Start dev server (localhost:3000)
+npm run build      # Build for production
+npm run lint       # Run ESLint
+npm start          # Start production server
+```
+
+## Architecture
+
+### Entry Point
+- `app/page.tsx` — renders `<GeminiChat />` only; all logic lives in components
+
+### Core Component: `components/gemini/GeminiChat.tsx`
+The root stateful component. Owns all state: sidebar open/close, active conversation, message list, typing indicator. Composes all other Gemini components.
+
+### Gemini Component Breakdown
+
+| Component | Purpose |
+|---|---|
+| `GeminiSidebar.tsx` | Left sidebar — logo, new chat button, conversation history list, bottom nav (Help/Activity/Extensions), user avatar |
+| `GeminiTopbar.tsx` | Top bar — hamburger toggle, model selector dropdown (Gemini 2.0 Flash etc.), settings + avatar |
+| `GeminiWelcome.tsx` | Empty state — gradient greeting, 4 suggestion cards |
+| `GeminiMessage.tsx` | Single message — user bubble (right-aligned, bg pill) vs AI response (left-aligned, ◆ icon, no bubble, markdown-like rendering) |
+| `GeminiInput.tsx` | Bottom input — rounded container, attachment icon, auto-resize textarea, mic + send button |
+| `GeminiTypingIndicator.tsx` | Animated dots shown while AI is "responding" |
+| `ExtensionsPanel.tsx` | Extensions panel UI |
+
+### Styling
+- Tailwind CSS 4 with `@tailwindcss/postcss`
+- Dark theme only — hardcoded color palette:
+  - Page bg: `#131314`, Sidebar: `#1e1e1e`, Hover/card: `#2a2a2a`, Border: `#3a3a3a`
+  - Primary text: `#e3e3e3`, Muted: `#9aa0a6`
+  - Accent gradient: `from-[#4285f4] to-[#a855f7]`
+- `components/ui/` — standard shadcn/ui primitives (do not modify directly)
+
+### Key Patterns
+- All components use `"use client"` — no server components beyond the page shell
+- `cn()` from `@/lib/utils` for conditional class merging
+- No external API calls — all data is mock/local state in `GeminiChat.tsx`
+
+## Sync Workflow
+This repo is auto-synced from v0.app. Changes made on v0.app are pushed here automatically, then deployed to Vercel. Manual edits here may be overwritten on the next v0 sync.