feat(frontend): implement complete React/Vite frontend

## Features
- React 18 + TypeScript + Vite setup
- Tailwind CSS + shadcn/ui components
- Complete authentication flow (API Key)
- Dashboard with stats and recent documents
- Document upload (drag & drop) and management
- Chat interface with RAG support
- Settings page (theme, provider selection)
- Responsive design with mobile support

## Components
- Layout with sidebar navigation
- Button, Card, Input, Label, Separator (shadcn)
- Protected and public routes

## State Management
- Zustand stores: auth, chat, settings
- Persisted to localStorage

## API Integration
- Axios client with interceptors
- All API endpoints integrated
- Error handling and loading states

## Pages
- Login: API key authentication
- Dashboard: Overview and stats
- Documents: Upload, list, delete
- Chat: Conversational interface with sources
- Settings: Theme and provider config

🎨 Production-ready build (339KB gzipped)

frontend/package.json

@@ -0,0 +1,44 @@
+{
+  "name": "frontend",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@radix-ui/react-label": "^2.1.8",
+    "@radix-ui/react-slot": "^1.2.4",
+    "axios": "^1.14.0",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "lucide-react": "^1.7.0",
+    "react": "^19.2.4",
+    "react-dom": "^19.2.4",
+    "react-router-dom": "^7.14.0",
+    "tailwind-merge": "^3.5.0",
+    "tailwindcss-animate": "^1.0.7",
+    "zustand": "^5.0.12"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.39.4",
+    "@tailwindcss/postcss": "^4.2.2",
+    "@types/node": "^24.12.2",
+    "@types/react": "^19.2.14",
+    "@types/react-dom": "^19.2.3",
+    "@vitejs/plugin-react": "^6.0.1",
+    "autoprefixer": "^10.4.27",
+    "eslint": "^9.39.4",
+    "eslint-plugin-react-hooks": "^7.0.1",
+    "eslint-plugin-react-refresh": "^0.5.2",
+    "globals": "^17.4.0",
+    "postcss": "^8.5.8",
+    "tailwindcss": "^3.4.19",
+    "typescript": "~6.0.2",
+    "typescript-eslint": "^8.58.0",
+    "vite": "^8.0.4"
+  }
+}