feat: reveal available updates beside speed

Collapse the header update action completely when no update exists and animate the green action outward to the left of the fixed bandwidth widget only after a real update is detected.

renderer/app.js

@@ -801,6 +801,7 @@ function _setHeaderUpdateLabel(text) {
 
 function _syncHeaderUpdateState() {
   const button = document.getElementById('headerUpdateBtn');
+  const slot = document.getElementById('headerUpdateSlot');
   const available = !!(_knownUpdateInfo && _knownUpdateInfo.available);
   const version = available ? String(_knownUpdateInfo.remoteVersion || '').replace(/^v/i, '') : '';
   const hint = _updateCheckBusy
@@ -808,10 +809,15 @@ function _syncHeaderUpdateState() {
     : available
       ? `Update v${version || 'unbekannt'} verfügbar. Klicken zum Installieren.`
       : 'Nach Aktualisierungen suchen';
+  if (slot) {
+    slot.classList.toggle('is-visible', available);
+    slot.setAttribute('aria-hidden', available ? 'false' : 'true');
+  }
   if (button) {
     button.classList.toggle('update-available', available);
     button.classList.toggle('is-checking', _updateCheckBusy);
-    button.disabled = _updateCheckBusy;
+    button.disabled = !available || _updateCheckBusy;
+    button.tabIndex = available ? 0 : -1;
     button.setAttribute('aria-busy', _updateCheckBusy ? 'true' : 'false');
     button.setAttribute('aria-label', hint);
     button.title = hint;
@@ -843,6 +849,7 @@ async function requestUpdateCheck({ forceRefresh = true } = {}) {
     } else if (result && result.error) {
       showCopyToast('Updateprüfung fehlgeschlagen');
     } else {
+      _knownUpdateInfo = null;
       showCopyToast('Kein Update verfügbar');
     }
     return result;