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.

backend/services/memberService.js

@@ -363,10 +363,10 @@ class MemberService {
             }
 
             if (!imageRecord) {
-                // Get latest image (highest sortOrder, then highest id)
+                // Ohne explizite imageId immer das Primärbild liefern (kleinste sortOrder).
                 imageRecord = await MemberImage.findOne({
                     where: { memberId },
-                    order: [['sortOrder', 'DESC'], ['id', 'DESC']]
+                    order: [['sortOrder', 'ASC'], ['id', 'ASC']]
                 });
             }
 
@@ -1666,12 +1666,12 @@ class MemberService {
                     continue;
                 }
 
-                const latestImage = this._selectLatestImage(images);
-                if (!latestImage) {
+                const primaryImage = this._selectPrimaryImage(images);
+                if (!primaryImage) {
                     continue;
                 }
 
-                const fileName = latestImage.fileName || `${latestImage.id}.jpg`;
+                const fileName = primaryImage.fileName || `${primaryImage.id}.jpg`;
                 const filePath = path.join('images', 'members', String(member.id), fileName);
                 if (!fs.existsSync(filePath)) {
                     continue;
@@ -1847,24 +1847,19 @@ class MemberService {
         };
     }
 
-    _selectLatestImage(images) {
+    _selectPrimaryImage(images) {
         if (!Array.isArray(images) || images.length === 0) {
             return null;
         }
         return images
             .slice()
             .sort((a, b) => {
-                const updatedA = a.updatedAt ? new Date(a.updatedAt).getTime() : 0;
-                const updatedB = b.updatedAt ? new Date(b.updatedAt).getTime() : 0;
-                if (updatedA !== updatedB) {
-                    return updatedB - updatedA;
-                }
                 const sortA = Number.isFinite(a.sortOrder) ? a.sortOrder : parseInt(a.sortOrder || '0', 10);
                 const sortB = Number.isFinite(b.sortOrder) ? b.sortOrder : parseInt(b.sortOrder || '0', 10);
                 if (sortA !== sortB) {
-                    return sortB - sortA;
+                    return sortA - sortB;
                 }
-                return (b.id || 0) - (a.id || 0);
+                return (a.id || 0) - (b.id || 0);
             })[0];
     }