Enhance AssistantPage with subtle scrollbar styling and improve voice preview handling

- Introduce a new utility class `.scrollbar-subtle` in `globals.css` for a refined scrollbar appearance.
- Update `AssistantPage` to apply the subtle scrollbar styling to overflow containers, enhancing the user interface.
- Reset message state and sequence in `useVoicePreview` hook upon resource release, ensuring clean state management.

frontend/src/hooks/use-voice-preview.ts

@@ -187,6 +187,8 @@ export function useVoicePreview(
     releaseResources();
     setLocalStream(null);
     setRemoteStream(null);
+    setMessages([]);
+    messageSeqRef.current = 0;
     setError(null);
     setMicWarning(null);
     setStatus("idle");