fix(chat): show visible error toast when no provider or model is selected

Sending a message without a provider/model selection failed silently:
only a console.warn was emitted and the UI gave no feedback. Surface a
toast (i18n key chat.chatInput.toast.noModelSelected, added to all 11
locales) so the user knows why the message was not sent.

packages/ui/src/components/chat/ChatInput.tsx

@@ -1002,6 +1002,7 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({ onOpenSettings, scrollTo
 
         if (!providerIdToSend || !modelIdToSend) {
             console.warn('Cannot send message: provider or model not selected');
+            toast.error(t('chat.chatInput.toast.noModelSelected'));
             return;
         }