Fix create trade offer flow and other related bugs

trades/mixins.py

@@ -0,0 +1,23 @@
+from cards.models import Card
+
+class TradeOfferContextMixin:
+    def get_context_data(self, **kwargs):
+        # Start with any context passed in.
+        context = kwargs.copy()
+        # Include available cards requirements for multiselect fields.
+        context.setdefault("cards", Card.objects.all().order_by("name", "rarity_level"))
+        
+        # Provide friend_codes and selected_friend_code as in TradeOfferCreateView
+        friend_codes = self.request.user.friend_codes.all()
+        context["friend_codes"] = friend_codes
+        
+        if "initiated_by" in self.request.GET:
+            try:
+                selected_friend_code = friend_codes.get(pk=self.request.GET.get("initiated_by"))
+            except friend_codes.model.DoesNotExist:
+                selected_friend_code = self.request.user.default_friend_code or friend_codes.first()
+        else:
+            selected_friend_code = self.request.user.default_friend_code or friend_codes.first()
+        context["selected_friend_code"] = selected_friend_code
+        
+        return context