Enhance quota management and refactor configuration handling

- Introduced `QuotaExceeded` settings in configuration to handle quota limits more effectively.
- Added preview model switching logic to `Client` to automatically use fallback models on quota exhaustion.
- Refactored `APIHandlers` to leverage new configuration structure.
- Simplified server initialization and removed redundant `ServerConfig` structure.
- Streamlined client initialization by unifying configuration handling throughout the project.
- Improved error handling and response mechanisms in both streaming and non-streaming flows.

config.yaml

@@ -1,7 +1,10 @@
 port: 8317
-auth_dir: "~/.cli-proxy-api"
+auth-dir: "~/.cli-proxy-api"
 debug: true
 proxy-url: ""
-api_keys:
+quota-exceeded:
+  switch-project: true
+  switch-preview-model: true
+api-keys:
   - "12345"
   - "23456"