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/components/ImportModal.tsx

@@ -66,13 +66,11 @@ export function ImportModal({ onClose, onDone }: Props) {
         onClose();
       }
     } else {
-      if (result || importState.error) clearImport();
       onClose();
     }
   };
 
   const handleDone = () => {
-    clearImport();
     onDone();
   };