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/app/globals.css

@@ -232,3 +232,43 @@
     text-transform: uppercase;
   }
 }
+
+@layer utilities {
+  .scrollbar-subtle {
+    scrollbar-width: thin;
+    scrollbar-color: color-mix(
+        in srgb,
+        var(--muted-soft) 48%,
+        transparent
+      )
+      transparent;
+  }
+
+  .scrollbar-subtle::-webkit-scrollbar {
+    width: 10px;
+    height: 10px;
+  }
+
+  .scrollbar-subtle::-webkit-scrollbar-track {
+    background: transparent;
+  }
+
+  .scrollbar-subtle::-webkit-scrollbar-thumb {
+    background-color: color-mix(
+      in srgb,
+      var(--muted-soft) 42%,
+      transparent
+    );
+    border: 3px solid transparent;
+    border-radius: 999px;
+    background-clip: content-box;
+  }
+
+  .scrollbar-subtle::-webkit-scrollbar-thumb:hover {
+    background-color: color-mix(
+      in srgb,
+      var(--muted-soft) 64%,
+      transparent
+    );
+  }
+}