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/MobileNav.tsx

@@ -13,7 +13,7 @@ export function MobileNav() {
       {/* Hamburger button in header */}
       <button
         onClick={toggleSidebar}
-        className="fixed top-3 left-3 z-40 p-1.5 rounded bg-discord-bg-secondary text-discord-text-primary md:hidden"
+        className="fixed top-3 left-3 z-[120] p-1.5 rounded bg-surface-channel text-txt-primary md:hidden"
       >
         <svg width="24" height="24" viewBox="0 0 24 24" fill="currentColor">
           {sidebarOpen ? (
@@ -27,7 +27,7 @@ export function MobileNav() {
       {/* Backdrop when sidebar is open on mobile */}
       {sidebarOpen && (
         <div
-          className="fixed inset-0 bg-black/50 z-30 md:hidden"
+          className="fixed inset-0 bg-black/50 z-[35] md:hidden"
           onClick={toggleSidebar}
         />
       )}