feat: Add self-use mode for model ratio and price configuration

- Introduce `SelfUseModeEnabled` setting to allow flexible model ratio configuration
- Update error handling to provide more informative messages when model ratios are not set
- Modify pricing and relay logic to support self-use mode
- Add UI toggle for enabling self-use mode in operation settings
- Implement fallback mechanism for model ratios when self-use mode is enabled

setting/operation_setting.go

@@ -3,6 +3,7 @@ package setting
 import "strings"
 
 var DemoSiteEnabled = false
+var SelfUseModeEnabled = false
 
 var AutomaticDisableKeywords = []string{
 	"Your credit balance is too low",