fix(webui): convert WebM to WAV for Xiaomi MiMo ASR transcription

MiMo ASR (mimo-v2.5-asr) only accepts audio/wav, audio/mp3, and
audio/mpeg formats. Web browsers record in WebM/Opus by default,
causing the API to reject the payload with a transcription error.

This change adds a frontend WebM→WAV converter using the Web Audio API
(DecodeAudioData + PCM encoding) that activates only when the
configured transcription provider is 'xiaomi_mimo'. Other providers
are unaffected — they continue to receive the original browser format.

Tested and confirmed working on WebUI.

webui/src/components/thread/ThreadComposer.tsx

@@ -172,6 +172,7 @@ interface ThreadComposerProps {
   workspaceError?: string | null;
   onWorkspaceScopeChange?: (scope: WorkspaceScopePayload) => void;
   pendingQueueKey?: string | null;
+  transcriptionProvider?: string | null;
 }
 
 const COMMAND_ICONS: Record<string, LucideIcon> = {
@@ -782,6 +783,7 @@ export function ThreadComposer({
   workspaceError = null,
   onWorkspaceScopeChange,
   pendingQueueKey = null,
+  transcriptionProvider = null,
 }: ThreadComposerProps) {
   const { t } = useTranslation();
   const [value, setValue] = useState("");
@@ -1193,6 +1195,7 @@ export function ThreadComposer({
     onError: setVoiceError,
     onTranscript: appendTranscription,
     onTranscribeAudio,
+    wantsWav: transcriptionProvider === "xiaomi_mimo",
   });
 
   useEffect(() => {

webui/src/components/thread/ThreadShell.tsx

@@ -736,6 +736,7 @@ export function ThreadShell({
           workspaceError={workspaceError}
           onWorkspaceScopeChange={onWorkspaceScopeChange}
           pendingQueueKey={chatId}
+          transcriptionProvider={settingsSnapshot?.transcription?.provider}
         />
       ) : (
         <ThreadComposer
@@ -765,6 +766,7 @@ export function ThreadShell({
           workspaceScopeDisabled={workspaceScopeDisabled}
           workspaceError={workspaceError}
           onWorkspaceScopeChange={onWorkspaceScopeChange}
+          transcriptionProvider={settingsSnapshot?.transcription?.provider}
         />
       )}
     </>