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/data/app-data.ts

@@ -295,6 +295,10 @@ export const AppData = {
      * Most of a bot message read aloud; the rest is cut.
      */
     SpeechMaxChars: 4000,
+    /**
+     * What a speech model reads when a voice is tried in the model settings.
+     */
+    VoicePreviewText: "Hi there. This is how I'll sound when I read my replies aloud.",
     /**
      * Largest picture a bot or a workspace can have.
      */