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/ImagePreview.tsx

@@ -10,7 +10,7 @@ export function ImagePreview() {
 
   return (
     <div
-      className="fixed inset-0 z-[60] flex items-center justify-center bg-discord-bg-overlay animate-fade-in cursor-pointer"
+      className="fixed inset-0 z-[200] flex items-center justify-center bg-surface-overlay animate-fade-in cursor-pointer"
       onClick={closeImagePreview}
     >
       <button