feat(pwa): Offline-Banner in App-Shell, reminders.css lazy geladen

Zeigt automatisch wenn navigator.onLine === false.
Blendet sich aus sobald Verbindung wiederhergestellt.
reminders.css aus globalem <link> entfernt (wird lazy geladen).

public/styles/layout.css

@@ -1897,6 +1897,40 @@
   }
 }
 
+/* --------------------------------------------------------
+ * Offline-Banner
+ * -------------------------------------------------------- */
+.offline-banner {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  z-index: calc(var(--z-toast) + 1);
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+  padding: var(--space-2) var(--space-4);
+  background-color: var(--color-warning-light);
+  color: var(--color-warning);
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-medium);
+  border-bottom: 1px solid color-mix(in srgb, var(--color-warning) 20%, transparent);
+  animation: offline-banner-in 0.2s var(--ease-out);
+}
+
+.offline-banner[hidden] {
+  display: none;
+}
+
+@keyframes offline-banner-in {
+  from { opacity: 0; transform: translateY(-100%); }
+  to   { opacity: 1; transform: translateY(0); }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .offline-banner { animation: none; }
+}
+
 /* --------------------------------------------------------
  * Print-Styles
  * -------------------------------------------------------- */