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/components/ai-chat.module.css

@@ -128,3 +128,29 @@
 .conversationItemActive {
     background: var(--mantine-color-gray-light);
 }
+
+/* Pending messages queued by the user while a turn is still streaming. They
+   are sent automatically, FIFO, once the current turn finishes. */
+.queuedList {
+    padding-bottom: var(--mantine-spacing-xs);
+}
+
+.queuedItem {
+    background: var(--mantine-color-gray-light);
+    border-radius: var(--mantine-radius-sm);
+    padding: 4px 8px;
+}
+
+.queuedIcon {
+    flex: none;
+    color: var(--mantine-color-dimmed);
+}
+
+.queuedText {
+    flex: 1;
+    min-width: 0;
+    color: var(--mantine-color-dimmed);
+    white-space: pre-wrap;
+    overflow-wrap: break-word;
+    word-break: break-word;
+}