fix(tts): play() must return a promise that resolves when audio finishes

Without this, await audio.play() resolves immediately after source.start(),
so the playQueue chains the next sentence before the current one finishes,
causing overlapping / interrupted playback.

frontend/src/composables/useVoiceAudio.ts

@@ -39,11 +39,14 @@ export function useVoiceAudio() {
     currentSource = source
     playing.value = true
 
-    source.onended = () => {
-      playing.value = false
-      currentSource = null
-    }
-    source.start(0)
+    return new Promise<void>((resolve) => {
+      source.onended = () => {
+        playing.value = false
+        currentSource = null
+        resolve()
+      }
+      source.start(0)
+    })
   }
 
   function stop(): void {