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/ThemeToggle.tsx

@@ -0,0 +1,41 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+const STORAGE_KEY = "mcpedia-theme";
+
+/** Dark mode toggle. Defaults to system preference, persisted in localStorage. */
+export default function ThemeToggle() {
+  const [dark, setDark] = useState(false);
+
+  useEffect(() => {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    const initial =
+      stored === "dark" ||
+      (stored === null &&
+        window.matchMedia("(prefers-color-scheme: dark)").matches);
+    setDark(initial);
+    apply(initial);
+  }, []);
+
+  function apply(isDark: boolean) {
+    document.documentElement.classList.toggle("dark", isDark);
+  }
+
+  function toggle() {
+    const next = !dark;
+    setDark(next);
+    localStorage.setItem(STORAGE_KEY, next ? "dark" : "light");
+    apply(next);
+  }
+
+  return (
+    <button
+      onClick={toggle}
+      aria-label="Toggle dark mode"
+      className="text-sm border border-zinc-300 dark:border-zinc-700 rounded px-2 py-1 hover:bg-zinc-100 dark:hover:bg-zinc-800"
+    >
+      {dark ? "🌙" : "☀️"}
+    </button>
+  );
+}