Make conversation queues mutually exclusive

tests/dashboard/conversation-read-state.test.tsx

@@ -440,17 +440,21 @@ describe('application-level read state', () => {
     expect(await screen.findByRole('button', { name: /u\/carol/ })).toBeInTheDocument();
   });
 
-  it('explains rows hidden by the seller filter instead of silently dropping them', async () => {
+  it('keeps seller rows reachable through their dedicated queue', async () => {
     server.use(
       http.get('/api/conversations', () => HttpResponse.json([
         chatFixture({ username: 'alice' }),
         chatFixture({ username: 'seller', sellerDetected: true }),
       ])),
     );
+    const user = userEvent.setup();
     renderConversations();
 
     await screen.findByRole('button', { name: /u\/alice/ });
-    expect(screen.getByText('1 hidden by the seller filter.')).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Sellers, 1' })).toBeInTheDocument();
+    await user.click(screen.getByRole('button', { name: 'Sellers, 1' }));
+    expect(screen.getByRole('button', { name: /u\/seller/ })).toBeInTheDocument();
+    expect(screen.queryByRole('button', { name: /u\/alice/ })).not.toBeInTheDocument();
   });
 
   it('flags an unanswered conversation that has gone stale', async () => {