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/functions/frontend/explain-audio-error.ts

@@ -0,0 +1,34 @@
+type Params = {
+    /**
+     * The streaming audio route the player failed on.
+     */
+    src: string;
+    fallback?: string;
+};
+
+/**
+ * An `<audio>` only says it failed, so ask its route again for the
+ * reason: a failure answers in JSON, and audio coming back instead is
+ * dropped at once.
+ */
+export default async function explainAudioError({
+    src,
+    fallback = "Couldn't play it. Try again.",
+}: Params) {
+    const controller = new AbortController();
+
+    try {
+        const res = await fetch(src, { signal: controller.signal, cache: "no-store" });
+
+        if (!res.headers.get("content-type")?.includes("json")) {
+            controller.abort();
+            return fallback;
+        }
+
+        const json = await res.json();
+
+        return json?.msg || fallback;
+    } catch {
+        return fallback;
+    }
+}