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>

src/components/study/StudioShell.jsx

@@ -23,7 +23,7 @@ export default function StudioShell({ stage, loading = false, fixedHeight = fals
   const cycleTheme = () => setReaderTheme(THEME_ORDER[(THEME_ORDER.indexOf(readerTheme) + 1) % THEME_ORDER.length]);
 
   return (
-    <div className={`bg-[var(--r-bg)] font-sans text-[var(--r-text)] ${fixedHeight ? 'flex h-screen flex-col overflow-hidden' : 'min-h-screen'}`} style={themeVars(readerTheme)}>
+    <div className={`bg-[var(--r-bg)] font-sans text-[var(--r-text)] ${fixedHeight ? 'fixed inset-0 flex flex-col overflow-hidden' : 'min-h-dvh'}`} style={themeVars(readerTheme)}>
       <div className="safe-top z-30 shrink-0 border-b border-[var(--r-line)] bg-[var(--r-bar)]">
         <div className="mx-auto flex h-12 max-w-[1600px] items-center gap-3 px-3">
           <button type="button" onClick={goHome} aria-label="Back to home"