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/index.css

@@ -34,6 +34,7 @@ select {
 /* iPhone and iPad: don't inflate text when the phone is turned sideways, no grey flash on tap, and no
    double-tap-to-zoom delay on controls. */
 html {
+  scrollbar-gutter: stable; /* pages with and without a scrollbar line up the same */
   -webkit-text-size-adjust: 100%;
   text-size-adjust: 100%;
   -webkit-tap-highlight-color: transparent;