feat: add Next.js metrics dashboard for real-time visualization

Add a lightweight Next.js dashboard to visualize OpenProxy metrics in real-time. The dashboard provides comprehensive insights into LLM API usage, costs, and performance.

Features:
- Real-time metrics overview (requests, tokens, costs, response times)
- Model breakdown with usage statistics
- Hourly trends visualization with charts
- Recent requests table with detailed information
- Auto-refresh every 30 seconds
- Configurable time ranges (1h, 6h, 24h, 7d)

Technical details:
- Built with Next.js 14 and React 18
- Uses Recharts for data visualization
- Connects directly to PostgreSQL database
- Runs on port 3008 by default
- TypeScript for type safety
- Minimal dependencies for lightweight deployment

The dashboard complements the proxy server by providing a user-friendly interface for monitoring and analyzing LLM API usage patterns.

dashboard/.gitignore

@@ -0,0 +1,34 @@
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env*.local
+.env
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts