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/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}
         />
       )}
     </>