Add missing setSelectedAudioInput mutation to Vuex store

- Fixes runtime error when selecting audio input device in MediaRecord component
- Completes the audio device selection feature implementation

Author: yibeichan

src/store/store.js

@@ -206,6 +206,10 @@ const mutations = {
   setExport(state, value) {
     state.hasExport = value;
   },
+  // eslint-disable-next-line
+  setSelectedAudioInput(state, deviceId) {
+    state.selectedAudioInput = deviceId;
+  },
 };
 
 const actions = {