Tighten markdown list spacing around prose

Lists previously had a static marginVertical that stacked with
paragraph.marginBottom, producing 20px gaps and oversized whitespace
around paragraphs sandwiched between lists.

Spacing is now context-aware via the render rule: list -> list keeps
the normal paragraph gap (back-to-back lists read as continuous
content) while p <-> list transitions get a slightly larger gap (lists
read as their own section). Static margins can't satisfy both because
RN doesn't collapse margins, so the bullet_list / ordered_list rule
inspects the next sibling type and picks marginBottom accordingly.

packages/app/src/utils/markdown-list.ts

@@ -60,6 +60,22 @@ function parseOrderedListStart(node: MarkdownNode): number {
   return 1;
 }
 
+export function getMarkdownNextSiblingType(
+  node: MarkdownNode,
+  parent: unknown,
+): string | undefined {
+  const ancestors = toParentNodes(parent);
+  for (let i = ancestors.length - 1; i >= 0; i--) {
+    const ancestor = ancestors[i];
+    if (!Array.isArray(ancestor?.children)) continue;
+    const idx = ancestor.children.indexOf(node);
+    if (idx >= 0) {
+      return ancestor.children[idx + 1]?.type;
+    }
+  }
+  return undefined;
+}
+
 export function getMarkdownListMarker(
   node: MarkdownNode,
   parent: unknown,