fix: restore .fixed-top (dropped with helpers in phase 3)

The header navbar uses .fixed-top for its full-width fixed positioning
(left:0;right:0); it lived in bootstrap/helpers, which phase 3 removed.
.navbar.fixed only set position:fixed, so on scroll-up the bar shrank to
content width. Re-added .fixed-top to the hand-rolled layer.

css/styles.scss

@@ -155,6 +155,7 @@ $enable-negative-margins: true;
 .pb-5 { padding-bottom: 3rem !important; }
 .py-3 { padding-top: 1rem !important; padding-bottom: 1rem !important; }
 .d-flex { display: flex !important; }
+.fixed-top { position: fixed; top: 0; right: 0; left: 0; z-index: 1030; } // was in bootstrap/helpers (position); header navbar full-width fixed
 .align-items-end { align-items: flex-end !important; }
 .justify-content-center { justify-content: center !important; }
 .float-end { float: right !important; }