fix: code block toolbar z-index issue in right sidebar (#7376)

Closes: [BS-549](https://linear.app/affine-design/issue/BS-549/code-block-%E5%9C%A8%E5%8F%B3%E4%BE%A7%E8%BE%B9%E6%A0%8F%E6%9C%89-z-index-%E9%97%AE%E9%A2%98%E8%AF%AD%E8%A8%80%E7%9A%84%E4%BF%A1%E6%81%AF%E5%87%BA%E7%8E%B0%E5%9C%A8%E4%BA%86-tab-%E4%B8%8A%E9%9D%A2),[BS-600](https://linear.app/affine-design/issue/BS-600/the-code-toolbar-remains-visible-when-it-overflows-in-the-ai-panel)

packages/frontend/core/src/modules/right-sidebar/view/header.css.ts

@@ -8,6 +8,7 @@ export const header = style({
   alignItems: 'center',
   flexShrink: 0,
   padding: '0 16px',
+  zIndex: 1,
   gap: '12px',
   background: cssVar('backgroundPrimaryColor'),
   selectors: {