Put every preference in Settings; same header on every page

- Settings > Preferences: typeface, line spacing, verse numbers, highlight colour names,
  My Studies grid/list and sort, verse-of-the-day Bible and picture, commentary sources, study templates
- Header uses one fixed width and reserves scrollbar space so the nav sits in the same place on every page

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

src/components/study/PageShell.jsx

@@ -95,7 +95,7 @@ export default function PageShell({ title, onBack, actions, active = null, maxWi
     <div className="min-h-dvh bg-[var(--r-bg)] font-sans text-[var(--r-text)]" style={themeVars(readerTheme)}>
       <a href="#content" className="skip-link">Skip to content</a>
       <header className="safe-top sticky top-0 z-30 border-b border-[var(--r-line)] bg-[var(--r-bar)]">
-        <div className={`mx-auto flex h-12 ${maxWidth} items-center gap-2 px-3 sm:gap-4 sm:px-4`}>
+        <div className="mx-auto flex h-12 max-w-6xl items-center gap-2 px-3 sm:gap-4 sm:px-4">
           <button type="button" onClick={goHome} className="-m-2 flex shrink-0 items-center gap-2 p-2" aria-label="Bible Study home">
             <img src="/logo.svg" alt="" className="h-6 w-6" />
             <span className="hidden text-sm font-medium sm:inline">Bible Study</span>