refactor: centralize section config — remove all hardcoded section lists

- New packages/config/src/sections.ts: SECTIONS, SECTIONS_BY_ID, SECTION_IDS
  (no Node.js built-in imports — safe for client components)
- New package entrypoint: @mcpedia/config/sections (exports only sections.ts)
- Client components import from @mcpedia/config/sections (no node:fs in bundle)
- Server components import from @mcpedia/config (has env/fs, server-only)
- Removed hardcoded SECTIONS/SECTION_LABEL/SECTION_ICON from:
  - [section]/page.tsx (was inline array)
  - [section]/[...slug]/page.tsx (was hardcoded Record maps)
  - Sidebar.tsx (was inline array)
  - layout.tsx (was inline array)
  - page.tsx (was inline array)
  - create/page.tsx (was inline array via SECTIONS import)
- Added @mcpedia/config/sections entrypoint to package.json exports

apps/web/app/components/Sidebar.tsx

@@ -3,6 +3,7 @@
 import { useEffect, useState } from "react";
 import Link from "next/link";
 import { usePathname } from "next/navigation";
+import { SECTIONS } from "@mcpedia/config/sections";
 
 interface Doc {
   slug: string;
@@ -11,13 +12,6 @@ interface Doc {
   tags: string[];
 }
 
-const SECTIONS = [
-  { id: "docs", label: "Documentation", icon: "📄" },
-  { id: "writeups", label: "Writeups", icon: "📝" },
-  { id: "research", label: "Research", icon: "🔬" },
-  { id: "notes", label: "Notes", icon: "📌" },
-] as const;
-
 /**
  * Build a hierarchical folder tree from flat document slugs.
  * Each node is either a folder (has children) or a leaf doc.
@@ -132,7 +126,7 @@ function renderTreeNode(
       </Link>
       {node.children.length > 0 && (
         <ul className="mt-0.5">
-          {node.children.map((child) => renderTreeNode(child, 0, pathname))}
+          {node.children.map((child) => renderTreeNode(child, depth + 1, pathname))}
         </ul>
       )}
     </li>