Increase header logo size from 44px to 64px

Feedback that the banner logo was too small; the display size is
set entirely by CSS (source image and Hugo's resize pipeline both
have plenty of resolution headroom above 64px).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/css/main.css

@@ -94,7 +94,7 @@ header.site .brand {
   text-decoration: none;
 }
 header.site .brand img {
-  height: 44px;
+  height: 64px;
   width: auto;
   display: block;
 }