feat: 404 UI 추가

Author: JunilHwang

src/main.js

@@ -1084,6 +1084,41 @@ function main() {
     </div>
   `;
 
+  const _404_ = `
+    <main class="max-w-md mx-auto px-4 py-4">
+      <div class="text-center my-4 py-20 shadow-md p-6 bg-white rounded-lg">
+      <svg viewBox="0 0 320 180" xmlns="http://www.w3.org/2000/svg">
+        <defs>
+          <linearGradient id="blueGradient" x1="0%" y1="0%" x2="100%" y2="100%">
+            <stop offset="0%" style="stop-color:#4285f4;stop-opacity:1" />
+            <stop offset="100%" style="stop-color:#1a73e8;stop-opacity:1" />
+          </linearGradient>
+          <filter id="softShadow" x="-50%" y="-50%" width="200%" height="200%">
+            <feDropShadow dx="0" dy="2" stdDeviation="8" flood-color="#000000" flood-opacity="0.1"/>
+          </filter>
+        </defs>
+        
+        <!-- 404 Numbers -->
+        <text x="160" y="85" font-family="-apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif" font-size="48" font-weight="600" fill="url(#blueGradient)" text-anchor="middle">404</text>
+        
+        <!-- Icon decoration -->
+        <circle cx="80" cy="60" r="3" fill="#e8f0fe" opacity="0.8"/>
+        <circle cx="240" cy="60" r="3" fill="#e8f0fe" opacity="0.8"/>
+        <circle cx="90" cy="45" r="2" fill="#4285f4" opacity="0.5"/>
+        <circle cx="230" cy="45" r="2" fill="#4285f4" opacity="0.5"/>
+        
+        <!-- Message -->
+        <text x="160" y="110" font-family="-apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif" font-size="14" font-weight="400" fill="#5f6368" text-anchor="middle">페이지를 찾을 수 없습니다</text>
+        
+        <!-- Subtle bottom accent -->
+        <rect x="130" y="130" width="60" height="2" rx="1" fill="url(#blueGradient)" opacity="0.3"/>
+      </svg>
+      
+      <a href="/" data-link class="inline-block px-6 py-2 bg-blue-500 text-white rounded hover:bg-blue-600 transition-colors">홈으로</a>
+    </div>
+    </main>
+  `;
+
   document.body.innerHTML = `
     ${상품목록_레이아웃_로딩}
     <br />
@@ -1104,6 +1139,8 @@ function main() {
     ${상세페이지_로딩}
     <br />
     ${상세페이지_로딩완료}
+    <br />
+    ${_404_}
   `;
 }