fix(layout): restore mx-auto centering by scoping CSS reset to @layer base

Unlayered CSS always wins over Tailwind's @layer utilities, so the
bare * { margin: 0 } reset was overriding mx-auto everywhere. Moving
it into @layer base restores correct cascade order.

src/components/Footer.tsx

@@ -1,7 +1,7 @@
 export default function Footer() {
   return (
     <footer className="border-t border-[var(--color-border)] py-5 mt-8">
-      <div className="max-w-[740px] mx-auto px-6 flex items-center justify-between">
+      <div className="px-8 flex items-center justify-between">
         <span className="font-mono text-xs text-[var(--color-text-dim)]">
           &copy; {new Date().getFullYear()} Tyler Koenig
         </span>

src/components/Nav.tsx

@@ -13,7 +13,7 @@ export default function Nav() {
   const pathname = usePathname();
   return (
     <header className="sticky top-0 z-50 bg-[var(--color-surface)] border-b border-[var(--color-border)]">
-      <nav className="max-w-[740px] mx-auto px-6 h-11 flex items-center justify-between">
+      <nav className="max-w-[740px] mx-auto px-8 h-11 flex items-center justify-between">
         <Link
           href="/"
           className="font-mono text-sm font-bold text-[var(--color-text)] tracking-widest hover:text-[var(--color-text-label)]"