fix: remove body field badge, improve sidebar tree styling, use doc titles in homepage tree

- STANDARD_KEYS now includes 'body' to prevent it from rendering as a custom field badge
- Sidebar: tree-style with border-l + ml-2 indentation per level, section separators with border-b, better depth cues
- Homepage buildFolderTree: uses doc.title from DB (not path segments), so _index shows 'CTF Writeups' instead of 'Index'
- Sidebar: _index path segments no longer show leading-space titles (slugToTitle filters empty)

apps/web/app/[section]/[...slug]/page.tsx

@@ -325,7 +325,8 @@ export default async function DocPage({ params, searchParams }: DocPageProps) {
   // Any other key in the document metadata becomes a dynamic badge.
   const STANDARD_KEYS = new Set([
     "id", "slug", "title", "type", "section", "status",
-    "author", "tags", "path", "createdAt", "updatedAt", "extraFields",
+    "author", "tags", "path", "createdAt", "updatedAt",
+    "extraFields", "body",
   ]);
 
   // Edit mode: inline form