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/globals.css

@@ -1,5 +1,8 @@
 @import "tailwindcss";
 
+/* Enable class-based dark mode (toggled by ThemeToggle via .dark on <html>). */
+@custom-variant dark (&:where(.dark, .dark *));
+
 :root {
   --background: #ffffff;
   --foreground: #171717;
@@ -12,13 +15,6 @@
   --font-mono: var(--font-geist-mono);
 }
 
-@media (prefers-color-scheme: dark) {
-  :root {
-    --background: #0a0a0a;
-    --foreground: #ededed;
-  }
-}
-
 body {
   background: var(--background);
   color: var(--foreground);