feat: Relay 브랜드 로고·favicon 전면 적용

사이드바·인증 화면 'R' 텍스트 로고를 브랜드 더블 셰브론 마크(RelayMark)로 교체.
favicon을 SVG+PNG+ICO 세트와 웹 매니페스트로 정비(create-vue 기본 아이콘 대체), 가운데 셰브론을 키워 작은 크기 가독성 개선.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/assets/images/logo/svg/relay-app-icon.svg

@@ -1,13 +1,13 @@
 <svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512" role="img" aria-label="Relay app icon">
   <defs>
     <linearGradient id="relayGrad" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0" stop-color="#5b54f0"></stop>
-      <stop offset="1" stop-color="#4338ca"></stop>
+      <stop offset="0" stop-color="#5b54f0"/>
+      <stop offset="1" stop-color="#4338ca"/>
     </linearGradient>
   </defs>
-  <rect width="512" height="512" rx="143" fill="url(#relayGrad)"></rect>
-  <svg x="160" y="160" width="192" height="192" viewBox="0 0 32 32">
-    <path d="M8 8 L16 16 L8 24" fill="none" stroke="#ffffff" stroke-width="3.2" stroke-linecap="round" stroke-linejoin="round"></path>
-    <path d="M16 8 L24 16 L16 24" fill="none" stroke="#ffffff" stroke-width="3.2" stroke-linecap="round" stroke-linejoin="round"></path>
-  </svg>
-</svg>
+  <rect width="512" height="512" rx="120" fill="url(#relayGrad)"/>
+  <g fill="none" stroke="#ffffff" stroke-width="52" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M128 130 L256 256 L128 382"/>
+    <path d="M256 130 L384 256 L256 382"/>
+  </g>
+</svg>