fix(queue): clear the queue card after the last queued message is sent

When the server delivered the last item of a session's queue it deleted
the session entry and broadcast the update with an empty directory. The
UI keys its projection by directory, could not build the key, and dropped
the event, so the delivered message stayed in the "Queued messages" card
until the next full hydration.

The server now remembers a session's directory beyond the emptying of its
queue so that broadcast names it. The UI additionally treats an empty
session without a directory as "this session's queue is done" for every
projection keyed under that session id, which covers clients talking to
an older server.

Claude-Session: https://claude.ai/code/session_01VqV56Hez25hTxXH4ipJfzH

packages/ui/src/stores/messageQueueStore.server.test.ts

@@ -233,6 +233,23 @@ describe("server-owned message queue", () => {
     expect(useMessageQueueStore.getState().queuedMessages[key]?.map((m) => m.content)).toEqual(["newer"])
   })
 
+  test("an empty session without a directory still clears the projection it was keyed under", () => {
+    applyMessageQueueUpdatedEvent(updated(4, session([serverItem("q1", "queued")], "q1")), "runtime-a")
+    expect(useMessageQueueStore.getState().queuedMessages[key]).toHaveLength(1)
+    expect(useMessageQueueStore.getState().sendingIds[key]).toEqual(["q1"])
+
+    // A server that forgot the directory once the queue emptied.
+    applyMessageQueueUpdatedEvent(updated(5, { sessionId: "session-1", directory: "", items: [], sendingId: null }), "runtime-a")
+    expect(useMessageQueueStore.getState().queuedMessages[key]).toBe(undefined)
+    expect(useMessageQueueStore.getState().sendingIds[key]).toBe(undefined)
+
+    // Still never backwards, and never another runtime's projection.
+    applyMessageQueueUpdatedEvent(updated(6, session([serverItem("q2", "later")])), "runtime-a")
+    applyMessageQueueUpdatedEvent(updated(3, { sessionId: "session-1", directory: "", items: [], sendingId: null }), "runtime-a")
+    applyMessageQueueUpdatedEvent(updated(9, { sessionId: "session-1", directory: "", items: [], sendingId: null }), "runtime-b")
+    expect(useMessageQueueStore.getState().queuedMessages[key]?.map((m) => m.content)).toEqual(["later"])
+  })
+
   test("removeFromQueue and clearQueue update locally and tell the server", async () => {
     useMessageQueueStore.setState({ queuedMessages: { [key]: [{ id: "q1", content: "a", text: "a", createdAt: 1 }, { id: "q2", content: "b", text: "b", createdAt: 2 }] } })
     respond = () => json({ revision: 10, session: session([serverItem("q2", "b")]) })