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-.

backend/src/routes/import.ts

@@ -79,12 +79,6 @@ router.post(
           return;
         }
 
-        sseWrite(res, {
-          stage: 'import',
-          progress: 88,
-          message: 'Импорт в базу данных...',
-        });
-
         const result = await importStatement(converted);
         if (isValidationError(result)) {
           sseWrite(res, {