fix: remove will-change from nav-bottom and add iOS height fallback

will-change: transform on a position:relative flex child causes iOS WebKit
to composite the element at an incorrect position - creating a visible gap
below the nav bar. Remove it; CSS transform transitions use hardware
acceleration automatically on modern iOS without this hint.

Add -webkit-fill-available before 100dvh on .app-shell as a fallback for
iOS WebKit versions where 100dvh is computed slightly smaller than the
actual WKWebView height.

public/styles/layout.css

@@ -17,6 +17,10 @@
 .app-shell {
   display: flex;
   flex-direction: column;
+  /* -webkit-fill-available als iOS-Fallback: In manchen iOS-WebKit-Versionen
+   * ist 100dvh minimal kleiner als die tatsächliche WKWebView-Höhe, was zu einem
+   * sichtbaren Streifen unten führt. -webkit-fill-available füllt zuverlässig. */
+  height: -webkit-fill-available;
   height: 100dvh;
 }