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/app/components/Markdown.tsx

@@ -1,9 +1,12 @@
 import ReactMarkdown from "react-markdown";
+import rehypeSlug from "rehype-slug";
 
 export default function Markdown({ source }: { source: string }) {
   return (
     <div className="prose prose-zinc dark:prose-invert max-w-none [&_pre]:bg-zinc-100 dark:[&_pre]:bg-zinc-900 [&_pre]:p-3 [&_pre]:rounded [&_code]:font-mono [&_a]:underline">
-      <ReactMarkdown>{source}</ReactMarkdown>
+      <ReactMarkdown rehypePlugins={[rehypeSlug]}>
+        {source}
+      </ReactMarkdown>
     </div>
   );
 }