refactor(MemberService, MemberGalleryDialog): update image selection logic and enhance participant styling

- Refactored image selection logic in MemberService to prioritize primary images based on sortOrder.
- Updated MemberGalleryDialog styles for participant items, adding visual indicators and improved background colors for better user experience.

frontend/src/components/MemberGalleryDialog.vue

@@ -305,8 +305,29 @@ export default {
 }
 
 .gallery-member-item.is-participant {
-    background-color: transparent;
-    border: none;
+    box-shadow:
+        inset 0 0 0 4px #22c55e,
+        inset 0 0 0 999px rgba(34, 197, 94, 0.22);
+}
+
+.gallery-member-item.is-participant::after {
+    content: '✓';
+    position: absolute;
+    top: 8px;
+    right: 8px;
+    width: 26px;
+    height: 26px;
+    border-radius: 999px;
+    background: #16a34a;
+    color: #ffffff;
+    font-size: 16px;
+    font-weight: 800;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 12;
+    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.35);
+    pointer-events: none;
 }
 
 .gallery-member-image {
@@ -344,7 +365,9 @@ export default {
 }
 
 .gallery-member-item.is-participant .gallery-member-name {
-    color: #d9f7df;
+    color: #ffffff;
+    background-color: rgba(22, 163, 74, 0.82) !important;
+    font-weight: 700;
 }
 
 .gallery-member-placeholder {