feat: Phase 2 Aether Drift — chat component token migration + scroll fix

Migrate all 8 chat rendering components from Discord tokens to Aether Drift
design system: Message, MarkdownRenderer, MentionBadge, MentionPopover,
FriendsPage, TypingIndicator, Embed, ImagePreview. Eliminates ~83 discord-*
class references and ~24 hardcoded hex values from components/chat/.

Fix critical scroll regression from Phase 1 grid migration by adding explicit
grid-rows-[minmax(0,1fr)] and min-h-0 to AppLayout's grid container, restoring
the height constraint chain to MessageList's overflow-y-auto.

packages/web/src/components/chat/MentionBadge.tsx

@@ -25,13 +25,13 @@ export const MentionBadge = React.memo(function MentionBadge({ userId }: Mention
       const sorted = [...member.roles].sort((a, b) => b.position - a.position);
       color = sorted[0]!.color;
     } else if (ownerId && userId === ownerId) {
-      color = '#f23f43';
+      color = '#fda4af';
     } else {
-      color = '#5865f2'; // blurple default
+      color = '#7c6cf6'; // accent-primary default
     }
   } else {
     displayName = 'Unknown User';
-    color = '#a3a6aa'; // muted fallback
+    color = '#a0a0aa'; // text-secondary fallback
   }
 
   const handleClick = (e: React.MouseEvent) => {