feat: Phase 1 Aether Drift — mobile-first grid layout and token migration

Replace Discord flex layout with CSS Grid on desktop (312px sidebar +
1fr main) and overlay drawer on mobile. Migrate all discord-* class
references to Aether Drift design tokens across 15 files (~200 refs).

Layout: ServerSidebar becomes a fixed glass strip (md:glass-strip),
ChannelSidebar gets pl-[72px] offset, bottom bar is a glass-bubble
(z-105), MessageInput floats as absolute glass pill on desktop (z-110).
MemberSidebar/ActivityPanel hidden on mobile via hidden md:block.

Z-index stack: MobileNav backdrop z-35, sidebar z-40, glass strip z-100,
bottom bar z-105, input bubble z-110, hamburger z-120, profile popout
z-145, device panels z-150, modals z-200.

packages/web/src/components/layout/MemberListToggleButton.tsx

@@ -8,8 +8,8 @@ export function MemberListToggleButton() {
   return (
     <button
       onClick={toggleMemberList}
-      className={`w-8 h-8 flex items-center justify-center transition-colors rounded-[4px] hover:bg-discord-modifier-hover ${
-        memberListOpen ? 'text-discord-text-primary' : 'text-discord-text-muted hover:text-discord-text-secondary'
+      className={`w-8 h-8 flex items-center justify-center transition-colors rounded-[4px] hover:bg-interactive-hover ${
+        memberListOpen ? 'text-txt-primary' : 'text-txt-tertiary hover:text-txt-secondary'
       }`}
       title="Toggle Member List"
     >