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/shared/src/types.ts

@@ -524,6 +524,7 @@ export interface GroupDmUserIdentity {
 
 export interface CreateGroupDmRequest {
   users: GroupDmUserIdentity[];
+  fromDmChannelId?: string;
 }
 
 export interface CreateDmMessageRequest {