fix(pwa): install button appears in the guide once Chrome reports the app installable

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1736,6 +1736,18 @@ export const App: React.FC = () => {
               </div>
             ) : (
               <div className="space-y-3 pt-1">
+                {/* Chrome may report "installable" only after the guide opened (e.g. shortly after the
+                    old app was removed): then the real install is one tap away again */}
+                {deferredPwaPrompt && (
+                  <button
+                    type="button"
+                    onClick={handleInstallWebApp}
+                    className="btn-primary w-full h-12 text-sm font-semibold rounded-xl flex items-center justify-center gap-2 shadow-md active:scale-98 transition-transform cursor-pointer"
+                  >
+                    <Download className="w-4 h-4" />
+                    <span>Jetzt installieren</span>
+                  </button>
+                )}
                 <div className="p-3.5 rounded-xl bg-canvas border border-border space-y-2.5 text-xs text-main">
                   <p className="font-semibold text-accent flex items-center gap-1.5">
                     <Smartphone className="w-4 h-4" />