fix: disable overscroll bounce to keep header stationary

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

frontend/src/routes/layout.css

@@ -33,6 +33,10 @@
 
 /* Apply base styles directly to HTML elements */
 @layer base {
+  html {
+    overscroll-behavior: none;
+  }
+
   body {
     /* Use the theme variable directly using standard CSS syntax */
     background-color: var(--color-brand-sand);