fix: improve member highlight visibility on dark theme

Use custom .member-current class instead of tailwind bg-purple-50
to properly support dark theme. Current user now has visible
background on both light and dark themes.

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

frontend/src/index.css

@@ -181,6 +181,16 @@
   color: var(--text-secondary);
 }
 
+.member-current {
+  background-color: rgb(243, 232, 255);
+  border-color: rgb(216, 180, 254);
+}
+
+[data-theme="dark"] .member-current {
+  background-color: rgba(139, 92, 246, 0.2);
+  border-color: rgba(139, 92, 246, 0.5);
+}
+
 .animate-fadeIn {
   animation: fadeIn 0.2s ease-in-out;
 }