fix: use overflow-y:overlay so scrollbar doesn't shift layout

Removes scrollbar-gutter:stable which pushed content left.
overlay scrollbar floats over content with zero layout impact.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/index.css

@@ -64,15 +64,12 @@
 @layer base {
   html {
     overflow-x: clip;
-    scrollbar-gutter: stable;
+    overflow-y: overlay;
   }
   body {
     overflow-x: clip;
     @apply font-sans text-slate-800 bg-slate-50 antialiased;
   }
-  #root {
-    overflow-x: clip;
-  }
   
   h1, h2, h3, h4, h5, h6 {
     @apply font-serif text-primary-900;