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/layout/AppLayout.tsx

@@ -217,7 +217,7 @@ export function AppLayout() {
   }
 
   return (
-    <div className="h-screen flex flex-col md:grid md:grid-cols-[312px_1fr] bg-surface-base overflow-hidden">
+    <div className="h-screen flex flex-col md:grid md:grid-cols-[312px_1fr] md:grid-rows-[minmax(0,1fr)] bg-surface-base overflow-hidden">
       {/* Server sidebar - always visible on desktop, toggled on mobile */}
       <div className={`fixed inset-y-0 left-0 z-40 flex w-[312px] transition-transform duration-200 ${sidebarOpen ? 'translate-x-0' : '-translate-x-full md:translate-x-0'} md:static md:z-auto md:w-auto md:transform-none`}>
         <ServerSidebar />
@@ -225,7 +225,7 @@ export function AppLayout() {
       </div>
 
       {/* Main content area */}
-      <div className="flex-1 flex min-w-0 bg-surface-chat relative">
+      <div className="flex-1 flex min-w-0 min-h-0 bg-surface-chat relative">
         <MainContent />
         <RightPanel />
       </div>