fix : increase nav bar z index (#1625)

Increase nav bar z index to avoid it getting covered by popover

Author: wyhong3103

src/components/header.tsx

@@ -151,7 +151,7 @@ function Header(props: HTMLChakraProps<'header'>) {
       transition='box-shadow 0.2s, background-color 0.2s'
       pos='sticky'
       top='0'
-      zIndex='3'
+      zIndex='11'
       bg='white'
       _dark={{ bg: 'gray.800' }}
       left='0'