fix: isolate content layer from animated background to eliminate flickering

Root cause: backdrop-filter on header/nav resamples the animated
background 60fps, causing GPU compositing flicker (Chromium bug).

- Add isolation: isolate wrapper in AppShell to prevent backdrop-filter
  from seeing through to the animated background layer
- Add will-change: transform + translateZ(0) to BackgroundRenderer
  for stable GPU layer
- Replace contain: layout style with contain: content on main
- Replace transition-all with specific properties in Card component

src/components/data-display/Card/Card.tsx

@@ -11,7 +11,7 @@ const cardVariants = cva(
     'relative overflow-hidden',
     'border border-dark-700/40 bg-dark-900/70',
     'rounded-[var(--bento-radius)]',
-    'transition-all duration-200',
+    'transition-[border-color,background-color,box-shadow,transform,opacity] duration-200',
     // GPU acceleration
     'transform-gpu',
     // Glass border inset