Make RailsGoat logo clickable on unauthenticated pages

Changed the logo from a non-interactive <span> to a clickable <a> link
pointing to the login page for unauthenticated users. This provides a
consistent navigation pattern across authenticated and unauthenticated
states.

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

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

app/views/layouts/shared/_header.html.erb

@@ -72,9 +72,9 @@
     <div class="container-fluid h-100">
       <div class="row h-100 align-items-center">
         <div class="col-auto">
-          <span class="rg-brand">
+          <a href="<%= login_path %>" class="rg-brand">
             <i class="bi bi-shield-fill-exclamation"></i> RailsGoat
-          </span>
+          </a>
         </div>
 
         <div class="col"></div>