fix: hide FAB when virtual keyboard is open on mobile

Uses visualViewport resize event to detect keyboard state (viewport height
< 75% of window height). Sets body.keyboard-visible class; CSS hides
.fab and .page-fab via visibility:hidden on screens < 1024px.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/router.js

@@ -410,6 +410,18 @@ window.addEventListener('auth:expired', () => {
   navigate('/login');
 });
 
+// --------------------------------------------------------
+// Virtuelle Tastatur: FAB ausblenden wenn Keyboard offen
+// Erkennung via visualViewport — Höhe < 75% des Fensters = Keyboard aktiv.
+// Nur auf Mobilgeräten relevant (< 1024px), Desktop hat keine virtuelle Tastatur.
+// --------------------------------------------------------
+if (window.visualViewport) {
+  window.visualViewport.addEventListener('resize', () => {
+    const keyboardVisible = window.visualViewport.height < window.innerHeight * 0.75;
+    document.body.classList.toggle('keyboard-visible', keyboardVisible);
+  });
+}
+
 // --------------------------------------------------------
 // Initialisierung
 // --------------------------------------------------------

public/styles/layout.css

@@ -872,6 +872,15 @@
   }
 }
 
+/* FAB und page-fab ausblenden wenn virtuelle Tastatur offen (nur Mobile) */
+@media (max-width: 1023px) {
+  .keyboard-visible .fab,
+  .keyboard-visible .page-fab {
+    visibility: hidden;
+    pointer-events: none;
+  }
+}
+
 /* --------------------------------------------------------
  * Form-Elemente
  * -------------------------------------------------------- */