fix: complete display and channel group management

tests/viewers.test.ts

@@ -128,6 +128,34 @@ describe("viewers", () => {
     expect(store.viewers.size).toBe(1);
   });
 
+  it("moves an assigned display atomically when its channel group is deleted", async () => {
+    const original = (await store.listChannelGroups())[0];
+    if (!original) {
+      throw new Error("Expected the default group.");
+    }
+    const session = await openViewerSession({
+      at: new Date(clock),
+      deviceId: null,
+      newId,
+      store,
+      viewerKey: "kiosk-lobby-0003",
+    });
+    expect(original.channels.some((channel) => channel.id === session.channel.id)).toBe(true);
+    const fallbackGroupId = newId();
+    const fallbackChannelId = newId();
+    await store.createChannelGroup({
+      channelId: fallbackChannelId,
+      channelName: "Default",
+      groupId: fallbackGroupId,
+      name: "Foyer",
+    });
+
+    await store.deleteChannelGroup(original.group.id, fallbackChannelId);
+
+    expect(await store.findChannelGroup(original.group.id)).toBeUndefined();
+    expect((await store.findViewer(session.viewer.id))?.channelId).toBe(fallbackChannelId);
+  });
+
   it("stores only the safe device identifier a paired principal carries", async () => {
     const session = await openViewerSession({
       at: new Date(clock),