Adjust dark mode background for transcription button

Updated the transcription button's dark mode background to use a semi-transparent gray for improved visual consistency.

src/lib/components/chat/ChatWindow.svelte

@@ -653,7 +653,7 @@
 							{#if transcriptionEnabled}
 								<button
 									type="button"
-									class="btn absolute bottom-2 right-10 mr-1.5 size-8 self-end rounded-full border bg-white/50 text-gray-500 transition-none hover:bg-gray-50 hover:text-gray-700 dark:border-transparent dark:bg-gray-600 dark:text-gray-300 dark:hover:bg-gray-500 dark:hover:text-white sm:right-9 sm:size-7"
+									class="btn absolute bottom-2 right-10 mr-1.5 size-8 self-end rounded-full border bg-white/50 text-gray-500 transition-none hover:bg-gray-50 hover:text-gray-700 dark:border-transparent dark:bg-gray-600/50 dark:text-gray-300 dark:hover:bg-gray-500 dark:hover:text-white sm:right-9 sm:size-7"
 									disabled={isReadOnly}
 									onclick={() => {
 										isRecording = true;