feat: decorative gold side lines + frosted glass header

- Gold vertical lines on both sides (desktop 1280px+), fixed position,
  gradient fade at top/bottom, positioned at 3vw from edges
- Header light mode: frosted glass (bg-white/60 backdrop-blur-xl)
  with subtle bottom border, matching dark mode aesthetic

src/app/globals.css

@@ -26,6 +26,37 @@ body {
   overflow-x: hidden;
 }
 
+/* ===== Decorative side lines (desktop only) ===== */
+
+@media (min-width: 1280px) {
+  body::before,
+  body::after {
+    content: "";
+    position: fixed;
+    top: 0;
+    bottom: 0;
+    width: 1px;
+    z-index: 40;
+    pointer-events: none;
+    background: linear-gradient(
+      180deg,
+      transparent 0%,
+      rgba(201, 169, 110, 0.15) 15%,
+      rgba(201, 169, 110, 0.25) 50%,
+      rgba(201, 169, 110, 0.15) 85%,
+      transparent 100%
+    );
+  }
+
+  body::before {
+    left: clamp(16px, 3vw, 48px);
+  }
+
+  body::after {
+    right: clamp(16px, 3vw, 48px);
+  }
+}
+
 /* ===== Selection ===== */
 
 ::selection {

src/components/layout/Header.tsx

@@ -126,7 +126,7 @@ export function Header() {
     <header
       className={`fixed top-0 z-50 w-full transition-all duration-500 ${
         scrolled || menuOpen
-          ? "bg-white/90 shadow-none backdrop-blur-xl dark:bg-black/40"
+          ? "bg-white/60 backdrop-blur-xl border-b border-neutral-200/30 dark:bg-black/40 dark:border-white/[0.06]"
           : "bg-transparent"
       }`}
     >