fix(client): классификация 409-кодов + run-факт плумбинг (#488)

Потребляет реальный контракт #487 на клиенте (без выдумывания кодов):
- error-message.ts: ветки для 409 A_RUN_ALREADY_ACTIVE / SUPERSEDE_TARGET_MISMATCH
  / SUPERSEDE_TIMEOUT / SUPERSEDE_INVALID — человеческие тексты СТРОГО ДО generic-
  веток по статусу (иначе юзер видит сырой JSON {"code":"A_RUN_ALREADY_ACTIVE"});
- extractRunId(message): чтение runId из start-метаданных (зеркало
  extractServerChatId) — live-обновление run-факта для FSM;
- getRun(chatId): POST /ai-chat/run — first-class run-факт с сервера (init на
  маунте + verify после supersede-mismatch).

Плумбинг под FSM-обвязку коммитов 2–5. Тесты: классификатор (все 4 кода + order-
guard), extractRunId.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/error-message.ts

@@ -39,6 +39,44 @@ export function describeChatError(
     };
   }
 
+  // #488 commit 5: the #487 concurrency-gate / supersede 409s. These arrive as a
+  // ConflictException(object) body carrying a `code` (and statusCode 409). They
+  // MUST be classified by `code` STRICTLY BEFORE any generic status branch, or the
+  // user sees the raw JSON `{"code":"A_RUN_ALREADY_ACTIVE",…}`. The code strings
+  // are the real #487 server contract (ai-chat.controller.ts) — do not invent.
+  if (/"code"\s*:\s*"A_RUN_ALREADY_ACTIVE"/.test(msg)) {
+    return {
+      title: t("The agent is already answering"),
+      detail: t(
+        "This chat already has a run in progress. Wait for it to finish, or interrupt it and send now.",
+      ),
+    };
+  }
+  if (/"code"\s*:\s*"SUPERSEDE_TARGET_MISMATCH"/.test(msg)) {
+    return {
+      title: t("Couldn't interrupt — the run changed"),
+      detail: t(
+        "The run you tried to interrupt is no longer the active one. Check the latest answer and try again.",
+      ),
+    };
+  }
+  if (/"code"\s*:\s*"SUPERSEDE_TIMEOUT"/.test(msg)) {
+    return {
+      title: t("Couldn't interrupt in time"),
+      detail: t(
+        "The previous run didn't stop in time. Nothing was sent — try sending again.",
+      ),
+    };
+  }
+  if (/"code"\s*:\s*"SUPERSEDE_INVALID"/.test(msg)) {
+    return {
+      title: t("Couldn't interrupt that run"),
+      detail: t(
+        "The run to interrupt doesn't belong to this chat. Reload and try again.",
+      ),
+    };
+  }
+
   if (/"statusCode"\s*:\s*403\b/.test(msg)) {
     return {
       title: t("AI chat is disabled"),