chore: initial Next.js template scaffold

Next.js 16 + React 19 + TypeScript 5 + Tailwind CSS 4 + shadcn/ui.
Includes layout components, pages (home, 404, error, loading),
hooks, types, and documentation.

src/components/layout/footer.tsx

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+import { siteConfig } from "@/lib/site"
+
+export function Footer() {
+  return (
+    <footer className="border-t">
+      <div className="mx-auto flex h-14 max-w-5xl items-center justify-between px-4">
+        <p className="text-sm text-muted-foreground">
+          &copy; {new Date().getFullYear()} {siteConfig.name}
+        </p>
+        <p className="text-sm text-muted-foreground">
+          Built with{" "}
+          <a
+            href="https://nextjs.org"
+            className="underline underline-offset-4 hover:text-foreground"
+            target="_blank"
+            rel="noopener noreferrer"
+          >
+            Next.js
+          </a>{" "}
+          and{" "}
+          <a
+            href="https://ui.shadcn.com"
+            className="underline underline-offset-4 hover:text-foreground"
+            target="_blank"
+            rel="noopener noreferrer"
+          >
+            shadcn/ui
+          </a>
+        </p>
+      </div>
+    </footer>
+  )
+}

src/components/layout/header.tsx

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+import Link from "next/link"
+
+import { siteConfig, navLinks } from "@/lib/site"
+import { buttonVariants } from "@/components/ui/button"
+import { Sheet, SheetContent, SheetTrigger } from "@/components/ui/sheet"
+import { Menu } from "lucide-react"
+
+function DesktopNav() {
+  return (
+    <nav className="hidden gap-6 md:flex" aria-label="Main navigation">
+      {navLinks.map((item) => (
+        <Link
+          key={item.href}
+          href={item.href}
+          className="text-sm font-medium text-muted-foreground transition-colors hover:text-foreground"
+        >
+          {item.label}
+        </Link>
+      ))}
+    </nav>
+  )
+}
+
+function MobileNav() {
+  return (
+    <Sheet>
+      <SheetTrigger
+        className={buttonVariants({ variant: "outline", size: "icon" }) + " md:hidden"}
+        aria-label="Open menu"
+      >
+        <Menu className="size-5" />
+      </SheetTrigger>
+      <SheetContent side="right">
+        <nav className="mt-8 flex flex-col gap-4" aria-label="Mobile navigation">
+          {navLinks.map((item) => (
+            <Link
+              key={item.href}
+              href={item.href}
+              className="text-base font-medium text-muted-foreground transition-colors hover:text-foreground"
+            >
+              {item.label}
+            </Link>
+          ))}
+        </nav>
+      </SheetContent>
+    </Sheet>
+  )
+}
+
+export function Header() {
+  return (
+    <header className="sticky top-0 z-50 border-b bg-background/95 backdrop-blur-sm supports-backdrop-filter:bg-background/60">
+      <div className="mx-auto flex h-14 max-w-5xl items-center justify-between px-4">
+        <Link href="/" className="text-lg font-semibold tracking-tight">
+          {siteConfig.name}
+        </Link>
+        <DesktopNav />
+        <MobileNav />
+      </div>
+    </header>
+  )
+}

src/components/layout/index.ts

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+export { Header } from "./header"
+export { Footer } from "./footer"