Let admins hear a voice sample before picking it

The Speech section of every model settings form now has "Hear a
sample", which streams a short sentence (AppData.VoicePreviewText) in
the form's model and voice before they're saved, through
GET api/admin/harness/preview-voice (super admins and admins, that
model only via grabModelCandidate). pickVoice is shared with
speakText, and explainAudioError reads why a streamed player failed
for both read-aloud and the sample.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/harness/audio/speak-text.ts

@@ -61,7 +61,11 @@ export default async function speakText({
     return result;
 }
 
-function pickVoice({
+/**
+ * The voice to ask for: the picked one when the model has it (or the
+ * model was picked by hand), else the model's first, else OpenAI's.
+ */
+export function pickVoice({
     candidate,
     voice,
 }: {