style: adjust the tree structure symbols

Author: qorzj

src/ui/App.tsx

@@ -283,7 +283,7 @@ export function App({ projectRoot, version = "", onRestart }: AppProps): React.R
           reasoningEffort: selection.reasoningEffort,
         },
       };
-      const content = `/model\n⎿ Set model to ${selection.model}`;
+      const content = `/model\n└ Set model to ${selection.model}`;
 
       if (activeSessionId) {
         sessionManager.addSessionSystemMessage(activeSessionId, content, meta);

src/ui/MessageView.tsx

@@ -91,7 +91,7 @@ export function MessageView({ message, collapsed }: Props): React.ReactElement |
           <Box flexGrow={1} flexDirection="column">
             <Text color="cyan">/model</Text>
             <Text color="cyan">
-              ⎿ Set model to{" "}
+              └ Set model to{" "}
               <Text bold color="#229ac3">
                 {message.meta.modelConfig?.model}
               </Text>