fix: exempt existing DM members from friend check in createGroup

When converting a 1-on-1 DM to a group, the existing DM partner was
incorrectly required to be your friend. DMs don't require friendship,
so this check was over-strict. Added fromDmChannelId parameter to
createGroup — existing members of the source DM are exempt from the
friendship validation.

packages/web/src/components/modals/AddDmMemberModal.tsx

@@ -107,7 +107,7 @@ export function AddDmMemberModal() {
             homeInstance: f.homeInstance,
           })),
         ];
-        const newChannel = await api.dm.createGroup({ users });
+        const newChannel = await api.dm.createGroup({ users, fromDmChannelId: dmChannelId });
         addDmChannel(newChannel);
         closeModal();
         navigate(`/channels/@me/${newChannel.id}`);