fix: cancel streaming generation before clearing chat

Prevent race condition where _messages.clear() invalidates the
assistantIndex while the generation stream is still yielding tokens.
This could cause RangeError if user taps "Clear Chat" during streaming.

Now _clearChat() cancels the subscription and stops generation before
clearing the message list.

Author: priyavratuniyal

lib/screens/chat_screen.dart

@@ -145,6 +145,9 @@ class _ChatScreenState extends State<ChatScreen> {
   }
 
   void _clearChat() {
+    _generationSubscription?.cancel();
+    _generationSubscription = null;
+    gemmaService.stopGeneration();
     setState(() {
       _messages.clear();
     });