fix: parse JSON response in federation generate-key handler

api() returns a Response object; chain .then(res => res.json()) before
accessing data.key so the returned key is displayed in #federation-key-display
and settings-key-display--visible is applied.

muxplex/frontend/app.js

@@ -2348,7 +2348,9 @@ function bindStaticEventListeners() {
 
   // Multi-Device tab — federation generate key button
   on($('federation-generate-btn'), 'click', function() {
-    api('POST', '/api/federation/generate-key').then(function(data) {
+    api('POST', '/api/federation/generate-key')
+      .then(function(res) { return res.json(); })
+      .then(function(data) {
         var displayEl = $('federation-key-display');
         if (displayEl && data && data.key) {
           displayEl.textContent = data.key;