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/backgrounds/BackgroundRenderer.tsx

@@ -90,6 +90,8 @@ export function BackgroundRenderer() {
         opacity: effectiveConfig.opacity,
         filter: effectiveConfig.blur > 0 ? `blur(${effectiveConfig.blur}px)` : undefined,
         contain: 'strict',
+        willChange: 'transform',
+        transform: 'translateZ(0)',
       }}
     >
       <Suspense fallback={null}>