feat: update configuration for OpenAI and Anthropic endpoints

- Created a new .env.example file with default environment variables for PORT, OPENAI_UPSTREAM_URL, ANTHROPIC_UPSTREAM_URL, and DATABASE_URL.
- Updated .npmignore to exclude all .env files except .env.example.
- Revised CONTRIBUTING.md to simplify the contribution process and provide clearer setup instructions.
- Enhanced cost.ts with detailed type definitions and improved cost calculation logic.
- Updated proxy.ts to include new environment variables and improved logging functionality.
- Modified README.md to reflect new configuration instructions and usage examples.
- Removed unnecessary dashboard files and streamlined the project structure.

dashboard/package.json

@@ -10,6 +10,7 @@
     "lint": "next lint"
   },
   "dependencies": {
+    "dotenv": "^17.2.3",
     "next": "^14.2.35",
     "react": "^18.3.0",
     "react-dom": "^18.3.0",