Replace ASCII art banners with mobile-responsive headers

- Replace fixed-width ASCII banners with CSS-styled section headers
- Add responsive styling for mobile devices
- Update home page, event pages, header and footer layouts
- Improve mobile experience with scalable headers and gradient dividers

🤖 Generated with [Claude Code](https://claude.ai/code)

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

layouts/_partials/header.html

@@ -1,10 +1,6 @@
 {{ if .IsHome }}
 <h1>{{ site.Title }}</h1>
-<div class="ascii-border">
-<pre>
-+===================================================================+
-</pre>
-</div>
+<div class="header-divider"></div>
 {{ else }}
 <h2><a href="{{ "/" | relURL }}">{{ site.Title }}</a></h2>
 {{ end }}