test(friends): drop stale origin arg from addDmChannel assertion

The DM "Message button" test asserted addDmChannel was called with two
arguments — the channel and an empty-string origin — but the assertion
has been stale since commit 7f3ca4e ("route DM creation to home instance
with federated identity", 2026-04-01). That refactor made FriendsPage
always route DM creation through the home api client and dropped the
second argument from the addDmChannel call because the remote friend's
instanceOrigin no longer applies — home-created DMs don't need a
channelOriginMap entry (lookups default to '' for missing keys; remote-
delivered DMs still get their origin tagged by useWebSocket).

The two-arg assertion was introduced on 2026-03-25 (commit 277b69a)
against an intermediate form of the code that was later rewritten. Drop
the trailing '' so the assertion matches the current, intentional
one-arg call.

packages/web/src/components/chat/FriendsPage.test.tsx

@@ -322,7 +322,7 @@ describe('FriendsPage', () => {
       });
 
       await waitFor(() => {
-        expect(mockAddDmChannel).toHaveBeenCalledWith(expect.objectContaining({ id: 'dm-channel-99' }), '');
+        expect(mockAddDmChannel).toHaveBeenCalledWith(expect.objectContaining({ id: 'dm-channel-99' }));
       });
 
       await waitFor(() => {