Fix bug that background color of audio/video button doesn't change on click

PiperOrigin-RevId: 819459391

Author: google-genai-bot

src/app/components/chat-panel/chat-panel.component.scss

@@ -377,8 +377,8 @@
   caret-color: white;
 }
 
-.audio-rec-btn,
-.video-rec-btn {
+button.audio-rec-btn,
+button.video-rec-btn {
   background-color: var(--chat-card-background-color);
   &.recording {
     background-color: var(--chat-panel-eval-fail-color);