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/glass.css

@@ -349,7 +349,8 @@ textarea.form-input {
     transform var(--transition-base) var(--ease-out),
     background-color var(--transition-fast),
     box-shadow var(--transition-fast);
-  will-change: transform;
+  /* will-change: transform absichtlich weggelassen - auf iOS (WebKit) korrumpiert die
+   * dauerhafte GPU-Layer-Promotion eines position:relative Flex-Kinds dessen Position. */
 }
 
 .nav-bottom--hidden {