fix: restore lightbox fixed positioning on GitHub Pages site

The body > *:not(#bg-canvas) rule carried ID-level specificity (1,0,1)
which overrode .lightbox's position:fixed and z-index:1000, leaving the
overlay stuck in document flow and invisible. Excluding #lightbox from
that rule lets the lightbox CSS take effect correctly.

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

docs/style.css

@@ -28,7 +28,7 @@
     pointer-events: none;
 }
 
-body > *:not(#bg-canvas) {
+body > *:not(#bg-canvas):not(#lightbox) {
     position: relative;
     z-index: 1;
 }