Refactor configuration and key management

- Removed key management configuration from .env.example and related code.
- Updated Makefile to load environment variables for HOST and PORT.
- Modified main.go to handle request logging with a wait group for graceful shutdown.
- Simplified dashboard statistics handler to focus on key counts and request metrics.
- Removed key manager implementation and related interfaces.
- Updated proxy server to use atomic counters for round-robin key selection.
- Cleaned up unused types and configurations in types.go.
- Added package-lock.json for frontend dependencies.

package-lock.json

@@ -0,0 +1,6 @@
+{
+  "name": "gpt-load",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {}
+}