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/styles/globals.css

@@ -268,10 +268,10 @@
     }
   }
 
-  /* Optimize main scrollable content */
+  /* Main content — containment removed to avoid GPU layer conflicts
+     with isolation: isolate on content wrapper */
   main {
-    /* Prevent layout shifts during scroll */
-    contain: layout style;
+    contain: content;
   }
 
   /* Light theme - Champagne */