Stop the page extending under the status bar; use visible-viewport units

Drops viewport-fit=cover so the iPad/iPhone status bar is its own area and the system's
top-edge blur no longer covers the app bar. Fixed-height study stages are position:fixed so
they cannot scroll the document, and min-h-screen/100vh become dvh units.

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

index.html

@@ -2,7 +2,7 @@
 <html lang="en">
   <head>
     <meta charset="UTF-8" />
-    <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Bible Study Project</title>
 
     <!-- PWA manifest -->