fix: render animated background via portal at z-index:-1 to stop implicit compositing

Previous isolation:isolate fix didn't work because:
- position:fixed elements still see through isolation boundary
- backdrop-filter samples all rendered content regardless of stacking context
- The real issue: animated background at z-index:0 with will-change:transform
  forces Chrome to implicitly composite EVERY overlapping element

Fix: render BackgroundRenderer via createPortal on document.body with
z-index:-1, placing it below the root stacking context. This eliminates
implicit compositing entirely.

Also:
- Remove backdrop-blur-xl from desktop header (was resampling bg 60fps)
- Remove will-change:opacity from card ::after pseudo-element
- Replace transition-all with transition-colors on header buttons

src/components/backgrounds/BackgroundRenderer.tsx

@@ -1,4 +1,5 @@
 import { Suspense, useMemo } from 'react';
+import { createPortal } from 'react-dom';
 import { useQuery } from '@tanstack/react-query';
 import { brandingApi } from '@/api/branding';
 import type { AnimationConfig, BackgroundType } from '@/components/ui/backgrounds/types';
@@ -83,20 +84,25 @@ export function BackgroundRenderer() {
       ? reduceMobileSettings(effectiveConfig.settings)
       : effectiveConfig.settings;
 
-  return (
+  // Render via portal on document.body with z-index: -1.
+  // This places the animated background BELOW the root stacking context,
+  // preventing Chrome's implicit compositing from promoting every
+  // overlapping element to its own GPU layer (the root cause of flickering).
+  return createPortal(
     <div
-      className="pointer-events-none fixed inset-0 z-0"
+      className="pointer-events-none fixed inset-0"
       style={{
+        zIndex: -1,
         opacity: effectiveConfig.opacity,
         filter: effectiveConfig.blur > 0 ? `blur(${effectiveConfig.blur}px)` : undefined,
         contain: 'strict',
-        willChange: 'transform',
-        transform: 'translateZ(0)',
+        backfaceVisibility: 'hidden',
       }}
     >
       <Suspense fallback={null}>
         <Component settings={settings} />
       </Suspense>
-    </div>
+    </div>,
+    document.body,
   );
 }

src/components/layout/AppShell/AppShell.tsx

@@ -280,19 +280,16 @@ export function AppShell({ children }: AppShellProps) {
 
   return (
     <div className="min-h-screen">
-      {/* Animated background — on its own GPU layer, below everything */}
+      {/* Animated background renders via portal on document.body at z-index: -1 */}
       <BackgroundRenderer />
 
-      {/* Content layer — isolated from background to prevent backdrop-filter
-          from resampling the animated background every frame (Chromium bug) */}
-      <div className="relative z-10" style={{ isolation: 'isolate' }}>
       {/* Global components */}
       <WebSocketNotifications />
       <CampaignBonusNotifier />
       <SuccessNotificationModal />
 
       {/* Desktop Header */}
-        <header className="fixed left-0 right-0 top-0 z-50 hidden border-b border-dark-800/50 bg-dark-950/80 backdrop-blur-xl lg:block">
+      <header className="fixed left-0 right-0 top-0 z-50 hidden border-b border-dark-800/50 bg-dark-950/95 lg:block">
         <div className="mx-auto grid h-14 max-w-6xl grid-cols-[auto_1fr_auto] items-center gap-4 px-6">
           {/* Logo */}
           <Link to="/" className="flex items-center gap-2.5" onClick={handleNavClick}>
@@ -381,7 +378,7 @@ export function AppShell({ children }: AppShellProps) {
                 toggleTheme();
               }}
               className={cn(
-                  'rounded-xl border border-dark-700/50 bg-dark-800/50 p-2 text-dark-400 transition-all duration-200 hover:bg-dark-700 hover:text-accent-400',
+                'rounded-xl border border-dark-700/50 bg-dark-800/50 p-2 text-dark-400 transition-colors duration-200 hover:bg-dark-700 hover:text-accent-400',
                 !canToggleTheme && 'pointer-events-none invisible',
               )}
               title={isDark ? t('theme.light') || 'Light mode' : t('theme.dark') || 'Dark mode'}
@@ -395,7 +392,7 @@ export function AppShell({ children }: AppShellProps) {
                 haptic.impact('light');
                 logout();
               }}
-                className="rounded-xl border border-dark-700/50 bg-dark-800/50 p-2 text-dark-400 transition-all duration-200 hover:bg-dark-700 hover:text-accent-400"
+              className="rounded-xl border border-dark-700/50 bg-dark-800/50 p-2 text-dark-400 transition-colors duration-200 hover:bg-dark-700 hover:text-accent-400"
               title={t('nav.logout')}
             >
               <LogoutIcon className="h-5 w-5" />
@@ -436,6 +433,5 @@ export function AppShell({ children }: AppShellProps) {
         wheelEnabled={wheelEnabled}
       />
     </div>
-    </div>
   );
 }

src/styles/globals.css

@@ -400,7 +400,6 @@ img.twemoji {
     transition: opacity 0.3s ease;
     pointer-events: none;
     z-index: 10;
-    will-change: opacity;
   }
 
   .bento-card-hover:hover::after {