fix: verify member removal persistence

lib/services/collection_service.dart

@@ -374,11 +374,42 @@ class CollectionService {
       throw Exception('Collection owner cannot be removed.');
     }
 
-    await supabase
+    final targetMembership = await supabase
+        .from('collection_members')
+        .select('id, role')
+        .eq('collection_id', collectionId)
+        .eq('user_id', memberUserId)
+        .maybeSingle();
+
+    if (targetMembership == null) {
+      throw Exception('Member not found in this collection.');
+    }
+    if (targetMembership['role'] == 'owner') {
+      throw Exception('Collection owner cannot be removed.');
+    }
+
+    final deleted = await supabase
         .from('collection_members')
         .delete()
+        .eq('id', targetMembership['id'] as String)
+        .select('id');
+
+    if (deleted.isEmpty) {
+      throw Exception(
+        'Member could not be removed (blocked by database policy).',
+      );
+    }
+
+    final stillExists = await supabase
+        .from('collection_members')
+        .select('id')
+        .eq('collection_id', collectionId)
         .eq('user_id', memberUserId)
-      .eq('collection_id', collectionId);
+        .maybeSingle();
+
+    if (stillExists != null) {
+      throw Exception('Member removal did not persist. Please try again.');
+    }
   }
 
   /// Leave a collection (for non-owners).