Fix: Proxy uses arrayBuffer for audio/image responses (not text)

Binary data (MP3 audio) was corrupted by resp.text(). Now detects
content-type and uses arrayBuffer() for audio/* and image/* responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/api/vocabulary/[...path]/route.ts

@@ -20,10 +20,21 @@ async function proxyRequest(
       fetchOptions.body = await request.text()
     }
     const resp = await fetch(url, fetchOptions)
+    const contentType = resp.headers.get('Content-Type') || 'application/json'
+
+    // Binary responses (audio, images) must use arrayBuffer, not text
+    if (contentType.startsWith('audio') || contentType.startsWith('image')) {
+      const buffer = await resp.arrayBuffer()
+      return new NextResponse(buffer, {
+        status: resp.status,
+        headers: { 'Content-Type': contentType },
+      })
+    }
+
     const data = await resp.text()
     return new NextResponse(data, {
       status: resp.status,
-      headers: { 'Content-Type': resp.headers.get('Content-Type') || 'application/json' },
+      headers: { 'Content-Type': contentType },
     })
   } catch (e) {
     return NextResponse.json({ error: String(e) }, { status: 502 })