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/components.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "default",
+  "rsc": false,
+  "tsx": true,
+  "tailwind": {
+    "config": "tailwind.config.js",
+    "css": "src/index.css",
+    "baseColor": "slate",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils"
+  }
+}