Fix: use available Mistral model on OpenRouter

mistral-7b-instruct no longer exists on OpenRouter.
Switch to mistral-small-3.1-24b-instruct:free.

https://claude.ai/code/session_015z3yZxNNfXF63JuQDuPbEG

index.html

@@ -549,7 +549,7 @@ body {
     "use strict";
 
     var API_URL = "https://openrouter.ai/api/v1/chat/completions";
-    var MODEL = "mistralai/mistral-7b-instruct";
+    var MODEL = "mistralai/mistral-small-3.1-24b-instruct:free";
 
     /* --- RAG corpus --- */
     var CORPUS = [