fix chat ux and ai model

backend/src/main/java/com/petshop/backend/service/OpenRouterAiService.java

@@ -31,7 +31,7 @@ public class OpenRouterAiService {
     @Value("${openrouter.api-key:}")
     private String apiKey;
 
-    @Value("${openrouter.model:openai/gpt-oss-120b:free}")
+    @Value("${openrouter.model:google/gemma-4-31b-it:free}")
     private String model;
 
     private final String openRouterUrl = "https://openrouter.ai/api/v1/chat/completions";

backend/src/main/java/com/petshop/backend/service/OpenRouterService.java

@@ -24,7 +24,7 @@ public class OpenRouterService {
     @Value("${openrouter.api-key:}")
     private String apiKey;
 
-    @Value("${openrouter.model:meta-llama/llama-3.3-70b-instruct:free}")
+    @Value("${openrouter.model:google/gemma-4-31b-it:free}")
     private String model;
 
     private final ObjectMapper objectMapper = new ObjectMapper();