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/tsconfig.json

@@ -0,0 +1,7 @@
+{
+  "files": [],
+  "references": [
+    { "path": "./tsconfig.app.json" },
+    { "path": "./tsconfig.node.json" }
+  ]
+}