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/context/ImportContext.tsx

@@ -20,6 +20,9 @@ export interface ImportProgress {
 
 interface ImportContextValue {
   importState: ImportProgress;
+  showModal: boolean;
+  openModal: () => void;
+  closeModal: () => void;
   startImport: (file: File) => void;
   clearImport: () => void;
 }
@@ -35,8 +38,12 @@ const ImportContext = createContext<ImportContextValue | null>(null);
 
 export function ImportProvider({ children }: { children: ReactNode }) {
   const [importState, setImportState] = useState<ImportProgress>(INITIAL);
+  const [showModal, setShowModal] = useState(false);
   const runningRef = useRef(false);
 
+  const openModal = useCallback(() => setShowModal(true), []);
+  const closeModal = useCallback(() => setShowModal(false), []);
+
   const startImport = useCallback((file: File) => {
     if (runningRef.current) return;
     runningRef.current = true;
@@ -92,7 +99,9 @@ export function ImportProvider({ children }: { children: ReactNode }) {
   }, []);
 
   return (
-    <ImportContext.Provider value={{ importState, startImport, clearImport }}>
+    <ImportContext.Provider value={{
+      importState, showModal, openModal, closeModal, startImport, clearImport,
+    }}>
       {children}
     </ImportContext.Provider>
   );