Pin reader and tab bars to their own compositing layer

iOS Safari/PWA can strand fixed and sticky bars mid-screen after the
toolbar collapses or the page rubber-bands. Add a .fixed-bar class
(translateZ(0), will-change: transform) to the reader top/bottom bars
and the phone tab bar.

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

src/index.css

@@ -79,6 +79,14 @@ input,
 
 /* Top bars leave room for the status bar / notch when the app is installed or full-screen
    (env() is 0 in an ordinary browser tab). */
+/* iOS Safari/PWA can leave fixed and sticky bars stranded mid-screen after the toolbar collapses or the page
+   rubber-bands. Giving them their own compositing layer keeps them pinned to the viewport edge. */
+.fixed-bar {
+  transform: translateZ(0);
+  -webkit-transform: translateZ(0);
+  will-change: transform;
+}
+
 .safe-top {
   padding-top: env(safe-area-inset-top, 0px);
 }