fix: reopen result modal from progress bar, faster progress, handle LLM context error

- Move import modal visibility into ImportContext so the Layout
  progress pill can reopen the result dialog after the modal was closed.
- Raise LLM progress cap from 85% to 98% and drop the intermediate
  -import 88%- SSE event to eliminate the visual stall after LLM finishes.
- Detect LLM context-length errors (n_keep >= n_ctx) and surface a
  clear message instead of generic -Временная ошибка конвертации-.

frontend/src/pages/HistoryPage.tsx

@@ -18,6 +18,7 @@ import { TransactionTable } from '../components/TransactionTable';
 import { Pagination } from '../components/Pagination';
 import { EditTransactionModal } from '../components/EditTransactionModal';
 import { ImportModal } from '../components/ImportModal';
+import { useImport } from '../context/ImportContext';
 import { toISODate } from '../utils/format';
 
 const PARAM_KEYS = [
@@ -125,7 +126,7 @@ export function HistoryPage() {
   const [accounts, setAccounts] = useState<Account[]>([]);
   const [categories, setCategories] = useState<Category[]>([]);
   const [editingTx, setEditingTx] = useState<Transaction | null>(null);
-  const [showImport, setShowImport] = useState(false);
+  const { showModal: showImport, openModal: openImport, closeModal: closeImport, clearImport } = useImport();
 
   useEffect(() => {
     getAccounts().then(setAccounts).catch(() => {});
@@ -197,7 +198,8 @@ export function HistoryPage() {
   };
 
   const handleImportDone = () => {
-    setShowImport(false);
+    closeImport();
+    clearImport();
     fetchData();
     getAccounts().then(setAccounts).catch(() => {});
   };
@@ -208,7 +210,7 @@ export function HistoryPage() {
         <h1>История операций</h1>
         <button
           className="btn btn-primary"
-          onClick={() => setShowImport(true)}
+          onClick={openImport}
         >
           Импорт выписки
         </button>
@@ -262,7 +264,7 @@ export function HistoryPage() {
 
       {showImport && (
         <ImportModal
-          onClose={() => setShowImport(false)}
+          onClose={closeImport}
           onDone={handleImportDone}
         />
       )}