Add lead shortcode; make banner a full-bleed hero image

- lead: wraps content in a Bootstrap .lead paragraph (moved from the
  ctsmockup site, where {.lead} could not target a paragraph).
- banner: resolves src as a page/global resource or base-URL-aware
  link, and spans the full viewport width via symmetric
  calc(50% - 50vw) margins while page content stays in .container.
  Optional height crops via object-fit: cover. A banner that starts a
  page's content is pulled flush against the navbar.
- html { overflow-x: hidden } to absorb the 100vw scrollbar overhang.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/scss/hugo_bootstrap_theme.scss

@@ -15,6 +15,12 @@
   --bs-body-line-height: 1.5;
 }
 
+// Keep the full-bleed `.banner` from adding a horizontal scrollbar
+// (100vw is wider than the viewport when a vertical scrollbar is shown).
+html {
+  overflow-x: hidden;
+}
+
 body {
   font-family: var(--bs-font-sans-serif);
   line-height: var(--bs-body-line-height);
@@ -61,8 +67,23 @@ h1, h2, h3, h4, h5 {
   }
 }
 
-// Images
+// Full-bleed banner / hero image: spans the viewport width while the
+// surrounding page content stays within `.container`. Works from inside
+// any centred `.container` because the horizontal margins cancel out the
+// container offset (50% of the parent - 50% of the viewport).
 .banner {
+  display: block;
+  width: 100vw;
+  max-width: 100vw;
+  margin-left: calc(50% - 50vw);
+  margin-right: calc(50% - 50vw);
   margin-bottom: 2rem;
 }
 
+// When the banner is the first thing in the page body, pull it flush
+// against the navbar by cancelling the layout's top padding (`py-5`).
+.content-body > .banner:first-child,
+.content-body > p:first-child > .banner:only-child {
+  margin-top: -3rem;
+}
+