fix: 가상선박 아이콘 SDF halo 아티팩트 제거

- 모선 icon-halo-color 노란색 제거 (SDF 바운딩박스 채움 현상)
- halo 통일: rgba(0,0,0,0.6) / width 0.5

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

frontend/src/components/korea/FleetClusterLayer.tsx

@@ -771,8 +771,8 @@ export function FleetClusterLayer({ ships, analysisMap: analysisMapProp, onShipS
           }}
           paint={{
             'icon-color': ['get', 'color'],
-            'icon-halo-color': ['case', ['==', ['get', 'isParent'], 1], '#fbbf24', 'rgba(0,0,0,0.6)'],
+            'icon-halo-color': 'rgba(0,0,0,0.6)',
-            'icon-halo-width': ['case', ['==', ['get', 'isParent'], 1], 2, 0.5],
+            'icon-halo-width': 0.5,
             'text-color': ['get', 'color'],
             'text-halo-color': '#000000',
             'text-halo-width': 1,