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>

README.md

@@ -3,6 +3,37 @@
 Simple Bootstrap-based theme for Hugo.
 
 
+## Shortcodes
+
+### `lead`
+
+Wraps its content in a Bootstrap `.lead` paragraph (the Markdown
+attribute syntax `{.lead}` cannot target a paragraph):
+
+```
+{{< lead >}}
+An introductory paragraph that stands out.
+{{< /lead >}}
+```
+
+Pass extra classes with `class`, e.g. `{{< lead class="mb-5" >}}`.
+
+### `banner`
+
+A full-bleed banner / hero image: it spans the entire viewport width
+while the rest of the page stays within `.container`. Put it on the
+first line of a page's content to get a hero flush against the navbar.
+
+```
+{{< banner src="hero.png" alt="…" >}}
+```
+
+`src` is resolved as a page resource first, then a global resource
+(`assets/`), then as a plain link respecting the site's base URL.
+Optional params: `title`, `height` (a CSS length such as `60vh`, which
+crops the image with `object-fit: cover`) and `class`.
+
+
 ## Custom error page
 
 The theme has a custom 404 error page.

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;
+}
+

layouts/_shortcodes/banner.html

@@ -1,4 +1,36 @@
-{{ $src := .Get "src" }}
+{{- /*
-{{ $alt := .Get "alt" }}
+  Full-bleed banner / hero image.
-{{ $title := .Get "title" }}
+
-<img class="banner img-fluid" src="{{ $src }}" alt="{{ $alt }}" title="{{ $title }}" width="100%">
+  The image spans the entire viewport width while the surrounding page
+  content stays within `.container` (see the `.banner` rules in
+  hugo_bootstrap_theme.scss). Place the shortcode as the first line of a
+  page's content to get a hero that sits flush against the navbar.
+
+  Params:
+    src     image path - resolved as a page resource first, then a global
+            resource (assets/), then treated as a link/URL respecting the
+            site's base URL
+    alt     alternative text (default: empty)
+    title   optional title attribute
+    height  optional CSS length (e.g. "60vh"); crops the image with
+            object-fit: cover
+    class   optional extra CSS classes
+*/ -}}
+{{- $src := .Get "src" -}}
+{{- $alt := or (.Get "alt") "" -}}
+{{- $title := .Get "title" -}}
+{{- $height := .Get "height" -}}
+{{- $class := .Get "class" -}}
+{{- $url := $src -}}
+{{- with $.Page.Resources.GetMatch $src -}}
+  {{- $url = .RelPermalink -}}
+{{- else -}}
+  {{- with resources.Get $src -}}
+    {{- $url = .RelPermalink -}}
+  {{- else -}}
+    {{- $url = $src | relURL -}}
+  {{- end -}}
+{{- end -}}
+<img class="banner{{ with $class }} {{ . }}{{ end }}" src="{{ $url }}" alt="{{ $alt }}"
+  {{- with $title }} title="{{ . }}"{{ end }}
+  {{- with $height }} style="height:{{ . }};object-fit:cover"{{ end }}>

layouts/_shortcodes/lead.html

@@ -0,0 +1,2 @@
+{{ $additional_classes := .Get "class" }}
+<p class="lead {{ $additional_classes }}">{{ .Inner | safeHTML }}</p>