feat: add OI Low coin source and improve Mixed mode UI

- Add oi_low as independent source_type for short opportunities
- Redesign Mixed mode with card-based selector (2x2 grid)
- Show combination summary with total coin limit
- Support both Chinese and English languages
- Change default limits to 10 for OI Top and OI Low

store/strategy.go

@@ -252,7 +252,9 @@ func GetDefaultStrategyConfig(lang string) StrategyConfig {
 			UseAI500:   true,
 			AI500Limit: 10,
 			UseOITop:   false,
-			OITopLimit: 20,
+			OITopLimit: 10,
+			UseOILow:   false,
+			OILowLimit: 10,
 		},
 		Indicators: IndicatorConfig{
 			Klines: KlineConfig{