fix: add 2K image default pricing at 1.5x base price

Previously 2K images used the same base price as 1K ($0.134).
Now 2K uses 1.5x multiplier ($0.201), consistent with 4K using 2x ($0.268).

- Backend: add 2K size branch in getDefaultImagePrice
- Frontend: update 2K placeholder from 0.134 to 0.201
- Tests: update assertions for new 2K default price

backend/internal/service/billing_service.go

@@ -543,7 +543,10 @@ func (s *BillingService) getDefaultImagePrice(model string, imageSize string) fl
 		basePrice = 0.134
 	}
 
-	// 4K 尺寸翻倍
+	// 2K 尺寸 1.5 倍，4K 尺寸翻倍
+	if imageSize == "2K" {
+		return basePrice * 1.5
+	}
 	if imageSize == "4K" {
 		return basePrice * 2
 	}