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/Embed.tsx

@@ -42,10 +42,10 @@ export function Embed({ url }: EmbedProps) {
   if (isLoading || !metadata) return null;
 
   return (
-    <div className="mt-2 max-w-[520px] bg-discord-bg-secondary rounded-[4px] border-l-4 border-discord-bg-tertiary flex overflow-hidden">
+    <div className="mt-2 max-w-[520px] bg-surface-channel rounded-[4px] border-l-4 border-border-hard flex overflow-hidden">
       <div className="flex-1 p-3 min-w-0">
         {metadata.siteName && (
-          <div className="text-[12px] text-discord-text-normal font-medium mb-1 truncate">
+          <div className="text-[12px] text-txt-message font-medium mb-1 truncate">
             {metadata.siteName}
           </div>
         )}
@@ -54,13 +54,13 @@ export function Embed({ url }: EmbedProps) {
             href={url} 
             target="_blank" 
             rel="noopener noreferrer"
-            className="text-[16px] text-discord-text-link font-semibold hover:underline block mb-2"
+            className="text-[16px] text-txt-link font-semibold hover:underline block mb-2"
           >
             {metadata.title}
           </a>
         )}
         {metadata.description && (
-          <div className="text-[14px] text-discord-text-normal leading-[1.125rem]">
+          <div className="text-[14px] text-txt-message leading-[1.125rem]">
             {metadata.description}
           </div>
         )}