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/components/Layout.tsx

@@ -1,6 +1,70 @@
-import { useState, type ReactNode } from 'react';
+import { useState, useEffect, useRef, useCallback, type ReactNode } from 'react';
 import { NavLink } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
+import { useImport } from '../context/ImportContext';
+
+function ImportProgressBar() {
+  const { importState, clearImport } = useImport();
+  const [visible, setVisible] = useState(false);
+  const hideTimerRef = useRef<ReturnType<typeof setTimeout> | undefined>(undefined);
+
+  const isActive = importState.active;
+  const isDone = importState.stage === 'done';
+  const isError = importState.stage === 'error';
+  const showBar = isActive || isDone || isError;
+
+  useEffect(() => {
+    if (showBar) {
+      setVisible(true);
+      if (hideTimerRef.current) clearTimeout(hideTimerRef.current);
+    }
+    if (isDone || isError) {
+      hideTimerRef.current = setTimeout(() => {
+        setVisible(false);
+        clearImport();
+      }, 10_000);
+    }
+    return () => {
+      if (hideTimerRef.current) clearTimeout(hideTimerRef.current);
+    };
+  }, [showBar, isDone, isError, clearImport]);
+
+  const handleClick = useCallback(() => {
+    if (hideTimerRef.current) clearTimeout(hideTimerRef.current);
+    setVisible(false);
+    clearImport();
+  }, [clearImport]);
+
+  if (!visible) return null;
+
+  const barClass = isError
+    ? 'import-progress-bar import-progress-bar--error'
+    : isDone
+      ? 'import-progress-bar import-progress-bar--done'
+      : 'import-progress-bar';
+
+  const labelText = isError
+    ? `Ошибка импорта: ${importState.message}`
+    : isDone && importState.result
+      ? `Импорт завершён — ${importState.result.imported} операций`
+      : `${importState.message} ${importState.progress}%`;
+
+  return (
+    <div className={barClass}>
+      <div
+        className="import-progress-bar__fill"
+        style={{ width: `${isDone ? 100 : importState.progress}%` }}
+      />
+      <button
+        type="button"
+        className={`import-progress-label ${isDone || isError ? 'import-progress-label--clickable' : ''}`}
+        onClick={isDone || isError ? handleClick : undefined}
+      >
+        {labelText}
+      </button>
+    </div>
+  );
+}
 
 export function Layout({ children }: { children: ReactNode }) {
   const { user, logout } = useAuth();
@@ -10,6 +74,8 @@ export function Layout({ children }: { children: ReactNode }) {
 
   return (
     <div className="layout">
+      <ImportProgressBar />
+
       <button
         type="button"
         className="burger-btn"