feat(web): full layout overhaul — Linear design system

Complete UI/UX overhaul, not just style changes:

Layout:
- Dark-mode-first (near-black #08090a canvas, whiteOpacity borders)
- Sticky header with brand + nav + theme toggle
- Sticky sidebar (xl+) with hierarchical doc tree
- Main content in max-w-3xl centered column
- Inter font system (via @font-face + CSS vars)
- Font feature settings cv01/ss03 for Linear-geometric Inter

Components rewritten in Linear aesthetic:
- Homepage: editorial-style doc listing with tags, section headers
- Doc page: breadcrumb-style nav links, metadata bar (author/date/tags),
  clean prose, Related + History sections
- TOC: rehype-slug heading anchors + github-slugger matching
- Create/Edit: inline form with Linear-style inputs/buttons
- Login: centered card-style, brand-indigo CTA button
- Docs index: sectioned listing with tag previews
- Search: dark-themed search with result cards + snippets
- ThemeToggle: system-default + localStorage persistence

Typography:
- prose with custom Tailwind classes matching Linear's:
  headings #f7f8f8 font-light, body #d0d6e0,
  links #7170ff with hover #828fff,
  code blocks #191a1b bg with #23252a border

New deps: rehype-slug (heading anchors), github-slugger (TOC matching)
Files: layout.tsx (overhaul), page.tsx, create/, login/, docs/,
       [section]/[...slug]/page.tsx, Sidebar.tsx, ThemeToggle.tsx,
       TOC.tsx, DocForm.tsx, Markdown.tsx, Search page

apps/web/app/components/ThemeToggle.tsx

@@ -4,7 +4,7 @@ import { useEffect, useState } from "react";
 
 const STORAGE_KEY = "mcpedia-theme";
 
-/** Dark mode toggle. Defaults to system preference, persisted in localStorage. */
+/** Dark mode toggle — persisted in localStorage, defaults to system preference. */
 export default function ThemeToggle() {
   const [dark, setDark] = useState(false);
 
@@ -15,27 +15,22 @@ export default function ThemeToggle() {
       (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);
+    document.documentElement.classList.toggle("dark", 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"
+      className="text-[#d0d6e0] hover:text-[#f7f8f8] p-1 rounded transition-colors"
     >
-      {dark ? "🌙" : "☀️"}
+      {dark ? "☀️" : "🌙"}
     </button>
   );
 }