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>

layouts/_shortcodes/banner.html

@@ -1,4 +1,36 @@
-{{ $src := .Get "src" }}
-{{ $alt := .Get "alt" }}
-{{ $title := .Get "title" }}
-<img class="banner img-fluid" src="{{ $src }}" alt="{{ $alt }}" title="{{ $title }}" width="100%">
+{{- /*
+  Full-bleed banner / hero image.
+
+  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 }}>