Fix footer appearing mid-page when content is minimal

body lacked display:flex + flex-direction:column, so the existing
flex:1 on .lt-main had no effect. Error pages (404, 403) and any
page with little content showed the footer immediately after content
rather than pinned to the viewport bottom.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/base.css

@@ -217,6 +217,8 @@ body {
   min-height: 100vh;
   overflow-x: hidden;
   position: relative;
+  display: flex;
+  flex-direction: column;
 }
 
 a {