fix: improve mobile scroll performance and accessibility

Remove global smooth scroll (caused laggy feel), use solid header bg on
mobile instead of backdrop-blur, respect prefers-reduced-motion, and add
targeted smooth scroll only for anchor nav clicks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/ui/web/src/styles/globals.css

@@ -87,11 +87,22 @@
 @layer base {
   * {
     @apply border-border outline-ring/50;
-    overscroll-behavior: none;
   }
 
   html {
-    scroll-behavior: smooth;
+    overscroll-behavior: none;
+    scroll-behavior: auto;
+    -webkit-overflow-scrolling: touch;
+  }
+
+  @media (prefers-reduced-motion: reduce) {
+    *,
+    *::before,
+    *::after {
+      animation-duration: 0.01ms !important;
+      animation-iteration-count: 1 !important;
+      transition-duration: 0.01ms !important;
+    }
   }
 
   body {