feat(ai-chat): queue messages typed while the agent is streaming

Previously a message composed while the AI agent was streaming a reply was
silently dropped (the composer early-returned on isStreaming). Now such
messages are queued FIFO and sent automatically once the current turn
finishes cleanly.

- chat-input: submit() enqueues while streaming (via new onQueue prop) and
  sends otherwise; during streaming show a queue Send button (when text is
  present) alongside the Stop button; the textarea stays usable.
- chat-thread: per-conversation queue in local state (mirrored in a ref);
  flush the next message in onFinish ONLY on a clean finish - ai@6 useChat
  fires onFinish from a finally on Stop/disconnect/error too, where the queue
  must be preserved. Pending messages render as removable chips above the
  composer. Queue is cleared on chat switch (parent remount) and survives
  in-place new-chat id adoption.
- queue-helpers: pure FIFO helpers (enqueue/dequeue/removeQueuedById) + tests.
- i18n: add en-US/ru-RU keys (Queue message, Remove queued message,
  Send when the agent finishes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/queue-helpers.test.ts

@@ -0,0 +1,107 @@
+import { describe, it, expect } from "vitest";
+import {
+  enqueueMessage,
+  dequeue,
+  removeQueuedById,
+  type QueuedMessage,
+} from "./queue-helpers";
+
+describe("enqueueMessage", () => {
+  it("appends a message to the end of the queue", () => {
+    const queue: QueuedMessage[] = [{ id: "a", text: "first" }];
+    const next = enqueueMessage(queue, { id: "b", text: "second" });
+    expect(next).toEqual([
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+
+  it("does not mutate the input queue", () => {
+    const queue: QueuedMessage[] = [{ id: "a", text: "first" }];
+    enqueueMessage(queue, { id: "b", text: "second" });
+    expect(queue).toEqual([{ id: "a", text: "first" }]);
+  });
+});
+
+describe("dequeue", () => {
+  it("returns {head:null, rest:[]} for an empty queue", () => {
+    expect(dequeue([])).toEqual({ head: null, rest: [] });
+  });
+
+  it("returns the first item as head and the remainder as rest", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+      { id: "c", text: "third" },
+    ];
+    const { head, rest } = dequeue(queue);
+    expect(head).toEqual({ id: "a", text: "first" });
+    expect(rest).toEqual([
+      { id: "b", text: "second" },
+      { id: "c", text: "third" },
+    ]);
+  });
+
+  it("does not mutate the input queue", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ];
+    dequeue(queue);
+    expect(queue).toEqual([
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+});
+
+describe("removeQueuedById", () => {
+  it("removes the matching id and leaves the others", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+      { id: "c", text: "third" },
+    ];
+    const next = removeQueuedById(queue, "b");
+    expect(next).toEqual([
+      { id: "a", text: "first" },
+      { id: "c", text: "third" },
+    ]);
+  });
+
+  it("returns an equivalent list when the id is not present", () => {
+    const queue: QueuedMessage[] = [{ id: "a", text: "first" }];
+    expect(removeQueuedById(queue, "missing")).toEqual([
+      { id: "a", text: "first" },
+    ]);
+  });
+
+  it("does not mutate the input queue", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ];
+    removeQueuedById(queue, "a");
+    expect(queue).toEqual([
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+});
+
+describe("FIFO order", () => {
+  it("preserves order across enqueue -> dequeue", () => {
+    let queue: QueuedMessage[] = [];
+    queue = enqueueMessage(queue, { id: "1", text: "one" });
+    queue = enqueueMessage(queue, { id: "2", text: "two" });
+    queue = enqueueMessage(queue, { id: "3", text: "three" });
+
+    const order: string[] = [];
+    while (queue.length > 0) {
+      const { head, rest } = dequeue(queue);
+      if (head) order.push(head.text);
+      queue = rest;
+    }
+    expect(order).toEqual(["one", "two", "three"]);
+  });
+});