[fix](trx-frontend-http): handle plain GET on /audio endpoint

Return 204 for non-WebSocket GET requests to /audio instead of
letting actix-ws reject with 400. This allows the browser's audio
availability probe to work correctly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/src/audio.rs

@@ -50,6 +50,11 @@ pub async fn audio_ws(req: HttpRequest, body: web::Payload) -> Result<HttpRespon
         return Ok(HttpResponse::NotFound().body("audio not enabled"));
     };
 
+    // Plain GET probe (no WebSocket upgrade) — return 204 to signal audio is available
+    if !req.headers().contains_key("upgrade") {
+        return Ok(HttpResponse::NoContent().finish());
+    }
+
     let mut rx_sub = ch.rx.subscribe();
     let tx_sender = ch.tx.clone();
     let mut info_rx = ch.info.clone();