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/vite.config.ts

@@ -0,0 +1,22 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+import path from 'path'
+
+// https://vite.dev/config/
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+  server: {
+    port: 3000,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8000',
+        changeOrigin: true,
+      },
+    },
+  },
+})