fix(map): S57 ENC sprite URL에 origin 프리픽스 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/common/components/map/S57EncOverlay.tsx

@@ -158,7 +158,10 @@ export function S57EncOverlay({ visible }: S57EncOverlayProps) {
 
       // sprite 등록 (중복 방지)
       if (!hasSprite(map, ENC_SPRITE_ID)) {
-        map.addSprite(ENC_SPRITE_ID, `${PROXY_PREFIX}/sprite/sprite`);
+        const spriteUrl = PROXY_PREFIX.startsWith('http')
+          ? `${PROXY_PREFIX}/sprite/sprite`
+          : `${window.location.origin}${PROXY_PREFIX}/sprite/sprite`;
+        map.addSprite(ENC_SPRITE_ID, spriteUrl);
       }
 
       // sources 등록 (타일 URL을 프록시로 치환)