lotus: don't call AudioContext.setSinkId with an undefined device id

audioOutputId comes from `useMediaDevices().audioOutput.selected$?.id`, which is
undefined until a device is selected. On the Tauri desktop webview the observable
emits undefined first, so setSinkId(undefined) threw "The provided value is not
of type 'AudioSinkOptions'" on every call join (repeatedly). Guard on a string
(the default device is the empty string, still valid).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/useAudioContext.tsx

@@ -178,7 +178,13 @@ export function useAudioContext<S extends string>(
     if (
       audioContext &&
       "setSinkId" in audioContext &&
-      !controlledAudioDevices
+      !controlledAudioDevices &&
+      // Skip until a device is actually selected. audioOutputId is undefined
+      // before MediaDevices resolves (e.g. on the Tauri desktop webview, where
+      // the selected$ observable emits undefined first); setSinkId(undefined)
+      // throws "The provided value is not of type 'AudioSinkOptions'". The
+      // default device is represented by the empty string, which is still valid.
+      typeof audioOutputId === "string"
     ) {
       // https://developer.mozilla.org/en-US/docs/Web/API/AudioContext/setSinkId
       // @ts-expect-error - setSinkId doesn't exist yet in types, maybe because it's not supported everywhere.