feat(MemberTransferService, MemberTransferDialog): clarify member transfer process and enhance UI feedback

- Updated MemberTransferService to explicitly load only active members with testMembership = false, improving clarity in the transfer logic.
- Added a UI hint in MemberTransferDialog to inform users that only active members will be transferred, enhancing user experience and transparency.

frontend/src/components/MemberTransferDialog.vue

@@ -23,6 +23,7 @@
       <div v-else>
         <div class="config-summary">
           <h4>{{ $t('memberTransferDialog.transferConfiguration') }}</h4>
+          <p class="active-members-hint">Es werden nur aktive Mitglieder übertragen.</p>
           <div class="summary-info">
             <div class="summary-row">
               <span class="summary-label">{{ $t('memberTransferDialog.server') }}:</span>
@@ -638,6 +639,15 @@ export default {
   margin-bottom: 1rem;
 }
 
+.active-members-hint {
+  margin: 0 0 1rem 0;
+  padding: 0.55rem 0.75rem;
+  border-left: 3px solid #2f7a5f;
+  background: #eef8f3;
+  color: #1f5f49;
+  font-size: 0.9rem;
+}
+
 .summary-row {
   display: flex;
   gap: 1rem;