feat: stream PDF import progress via SSE with global progress bar

Replace the synchronous PDF import with Server-Sent Events streaming
between the backend (LLM) and the browser. The user can now close the
import modal and continue working while the conversion runs — a fixed
progress bar in the Layout shows real-time stage and percentage.

frontend/src/api/import.ts

@@ -11,3 +11,74 @@ export async function importStatement(
     formData,
   );
 }
+
+export interface SseProgressEvent {
+  stage: 'pdf' | 'llm' | 'import';
+  progress: number;
+  message: string;
+}
+
+export interface SseDoneEvent {
+  stage: 'done';
+  progress: 100;
+  result: ImportStatementResponse;
+}
+
+export interface SseErrorEvent {
+  stage: 'error';
+  message: string;
+}
+
+export type SseEvent = SseProgressEvent | SseDoneEvent | SseErrorEvent;
+
+export async function importStatementStream(
+  file: File,
+  onEvent: (event: SseEvent) => void,
+): Promise<void> {
+  const formData = new FormData();
+  formData.append('file', file);
+
+  const res = await fetch('/api/import/statement', {
+    method: 'POST',
+    body: formData,
+    credentials: 'include',
+  });
+
+  if (!res.ok) {
+    let msg = 'Ошибка импорта';
+    try {
+      const body = await res.json();
+      if (body.message) msg = body.message;
+    } catch { /* use default */ }
+    onEvent({ stage: 'error', message: msg });
+    return;
+  }
+
+  const reader = res.body?.getReader();
+  if (!reader) {
+    onEvent({ stage: 'error', message: 'Streaming не поддерживается' });
+    return;
+  }
+
+  const decoder = new TextDecoder();
+  let buffer = '';
+
+  while (true) {
+    const { done, value } = await reader.read();
+    if (done) break;
+
+    buffer += decoder.decode(value, { stream: true });
+
+    const lines = buffer.split('\n');
+    buffer = lines.pop() ?? '';
+
+    for (const line of lines) {
+      const trimmed = line.trim();
+      if (!trimmed.startsWith('data: ')) continue;
+      try {
+        const parsed = JSON.parse(trimmed.slice(6)) as SseEvent;
+        onEvent(parsed);
+      } catch { /* skip malformed lines */ }
+    }
+  }
+}