fix(MarkdownRenderer): trim first/last child margins (#1308)

The MarkdownHeading component applies mt="5" to all headings, creating
unwanted top spacing when a heading is the first element rendered inside
the MarkdownRenderer. Add globalStyle rules on the root to strip
margin-top from the first child and margin-bottom from the last child.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: amir-zahedi

.changeset/trim-markdown-renderer-margins.md

@@ -0,0 +1,11 @@
+---
+'@autoguru/overdrive': patch
+---
+
+Remove excessive top/bottom margins on first/last children inside MarkdownRenderer
+
+The MarkdownHeading component applies `mt="5"` to all headings, which creates
+unwanted spacing when a heading is the first element rendered. This adds
+`globalStyle` rules to the MarkdownRenderer root to strip `margin-top` from
+the first child and `margin-bottom` from the last child, ensuring the
+component sits flush within its container.

lib/components/MarkdownRenderer/MarkdownRenderer.css.ts

@@ -1,4 +1,4 @@
-import { style } from '@vanilla-extract/css';
+import { globalStyle, style } from '@vanilla-extract/css';
 
 import { cssLayerComponent } from '../../styles/layers.css';
 import { overdriveTokens as tokens } from '../../themes/theme.css';
@@ -13,3 +13,11 @@ export const root = style({
 		},
 	},
 });
+
+globalStyle(`${root} > *:first-child`, {
+	marginTop: '0 !important',
+});
+
+globalStyle(`${root} > *:last-child`, {
+	marginBottom: '0 !important',
+});