fix whep player

src/hooks/useWhepPlayer.ts

@@ -157,7 +157,17 @@ export function useWhepPlayer({
         const location = res.headers.get("location") || res.headers.get("Location");
         if (location) {
           try {
-            sessionUrlRef.current = new URL(location, whepUrl).toString();
+            const baseUrl = new URL(whepUrl);
+            if (location.startsWith("/")) {
+              // MediaMTX can return absolute paths like "/cam1/whep/<id>".
+              // If WHEP is behind a prefix (e.g. "/mtx"), preserve that prefix for DELETE.
+              const parts = baseUrl.pathname.split("/").filter(Boolean);
+              const prefix = parts.slice(0, -2).join("/");
+              const normalized = prefix ? `/${prefix}${location}` : location;
+              sessionUrlRef.current = new URL(normalized, baseUrl.origin).toString();
+            } else {
+              sessionUrlRef.current = new URL(location, whepUrl).toString();
+            }
           } catch {
             sessionUrlRef.current = location;
           }