Fix brain image vertical centering with viewport-relative units

Changed offset from fixed -20px to -5vh for responsive centering
across different browser window sizes. The brain is now properly
centered in the available space above the footer navigation.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jeremymanning

css/style.css

@@ -997,7 +997,7 @@ body.homepage main {
 
 .brain-wrapper {
     position: absolute;
-    top: calc(50% - 20px); /* Offset up to visually center in the green area above the menu */
+    top: calc(50% - 5vh); /* Offset up to visually center in the green area above the menu */
     left: 50%;
     transform: translate(-50%, -50%);
     transition: left 0.27s ease, transform 0.27s ease, top 0.27s ease;