fix(css): scope --accent-underline to sponsor-become hover; restore --line-strong at rest

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

Author: vinta

website/static/style.css

@@ -424,7 +424,7 @@ kbd {
   font-weight: 700;
   letter-spacing: 0.01em;
   text-decoration: underline;
-  text-decoration-color: var(--accent-underline);
+  text-decoration-color: var(--line-strong);
   text-underline-offset: 0.2em;
   transition:
     color 180ms ease,
@@ -433,7 +433,7 @@ kbd {
 
 .sponsor-become:hover {
   color: var(--accent-deep);
-  text-decoration-color: var(--accent);
+  text-decoration-color: var(--accent-underline);
 }
 
 .sponsor-list {