Extract glass system into shared CSS utilities and design tokens

Centralize all hardcoded rgba color values into CSS custom properties
and extract glass/glow styles into reusable utility classes (.glass,
.glass-inner, .glow-border, .glow-border--animated) in main.css.

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

frontend/src/components/AttendeeList.vue

@@ -28,7 +28,7 @@ defineProps<{
 .attendee-list__heading {
   font-size: 0.75rem;
   font-weight: 700;
-  color: rgba(255, 255, 255, 0.5);
+  color: var(--color-text-muted);
   text-transform: uppercase;
   letter-spacing: 0.08em;
 }
@@ -44,7 +44,7 @@ defineProps<{
 
 .attendee-list__item {
   font-size: 0.95rem;
-  color: rgba(255, 255, 255, 0.85);
+  color: var(--color-text-soft);
   line-height: 1.4;
   overflow: hidden;
   text-overflow: ellipsis;
@@ -53,7 +53,7 @@ defineProps<{
 
 .attendee-list__empty {
   font-size: 0.9rem;
-  color: rgba(255, 255, 255, 0.5);
+  color: var(--color-text-muted);
   font-style: italic;
 }
 </style>