fix(dm): populate store before navigating to new group DM

addDmChannel must be called before navigate() so the route effect
finds the channel in the store immediately. Without this, the chat
view renders empty until the WS dm_channel_created event arrives.
Also show an error instead of silently returning when otherMember
cannot be determined.

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

@@ -18,6 +18,7 @@ export function AddDmMemberModal() {
   const modalData = useUIStore((s) => s.modalData);
   const closeModal = useUIStore((s) => s.closeModal);
   const dmChannels = useSpaceStore((s) => s.dmChannels);
+  const addDmChannel = useSpaceStore((s) => s.addDmChannel);
   const navigate = useNavigate();
   const myUserId = useAuthStore((s) => s.user?.id);
   const inputRef = useRef<HTMLInputElement>(null);
@@ -74,8 +75,12 @@ export function AddDmMemberModal() {
       if (!dmChannel.ownerId) {
         // 1-on-1 DM → create a new group DM with all 3 users
         const otherMember = dmChannel.members.find(m => m.id !== myUserId);
-        if (!otherMember) return;
+        if (!otherMember) {
+          setError('Could not determine the other member of this conversation.');
+          return;
+        }
         const newChannel = await api.dm.createGroup({ userIds: [otherMember.id, user.id] });
+        addDmChannel(newChannel);
         closeModal();
         navigate(`/channels/@me/${newChannel.id}`);
       } else {