fix: use smart default for displayField based on entityType

When displayField is omitted in award definitions, the backend now
selects the appropriate default field based on the entityType:
- dxcc → entity (country name)
- state → state
- grid → grid (4-character)
- callsign → callsign

Previously, it used a fixed fallback order that prioritized entity
over other fields, causing grid-based awards to show DXCC names.

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/services/awards.service.js

@@ -896,7 +896,19 @@ export async function getAwardEntityBreakdown(userId, awardId) {
       }
       displayName = String(rawValue || entity);
     } else {
-      displayName = qso.entity || qso.state || qso.grid || qso.callsign || String(entity);
+      // Smart default based on entityType when displayField is not specified
+      const defaultDisplayField = {
+        'dxcc': 'entity',
+        'state': 'state',
+        'grid': 'grid',
+        'callsign': 'callsign'
+      }[rules.entityType] || 'entity';
+
+      let rawValue = qso[defaultDisplayField];
+      if (defaultDisplayField === 'grid' && rawValue && rawValue.length > 4) {
+        rawValue = rawValue.substring(0, 4);
+      }
+      displayName = String(rawValue || entity);
     }
 
     if (!slotMap.has(slotKey)) {