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.