fix(design): scope .bg-glow::before selector to .puro-page

One missed prefix from the automated transform. Aligns with the
scoping contract established in 41664efe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/assets/puro.css

@@ -92,7 +92,7 @@
   z-index: 0;
   overflow: hidden;
 }
-.bg-glow::before,
+.puro-page .bg-glow::before,
 .puro-page .bg-glow::after {
   content: "";
   position: absolute;