feat(web): Phase 11 UI/UX — TOC, dark mode toggle, /docs index

- Install rehype-slug (proper heading anchors) + github-slugger (matching TOC)
- TOC component: auto-generated from h2/h3 headings, clickable anchors
- Dark mode toggle: ThemeToggle (localStorage + system default), class-based
- /docs index page (lists all docs by section)
- Markdown.tsx uses rehype-slug for stable heading ids
- globals.css: @custom-variant dark (.dark class) for class-based dark mode
- layout.tsx: nav includes ThemeToggle

Note: per user instruction, installed real deps (rehype-slug, github-slugger,
@types/hast) instead of hacky inline any-cast hacks.

apps/web/package.json

@@ -12,13 +12,16 @@
   "dependencies": {
     "@mcpedia/config": "workspace:*",
     "@mcpedia/core": "workspace:*",
+    "github-slugger": "^2.0.0",
     "next": "16.3.1",
     "react": "19.2.8",
     "react-dom": "19.2.8",
-    "react-markdown": "^9.0.0"
+    "react-markdown": "^9.0.0",
+    "rehype-slug": "^6.0.0"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
+    "@types/hast": "^3.0.5",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",