Revert SSE streaming for PDF import, use synchronous flow

SSE streaming added unnecessary complexity and latency due to
buffering issues across Node.js event loop, Nginx proxy, and
Docker layers. Reverted to a simple synchronous request/response
for PDF conversion. Kept extractLlmErrorMessage for user-friendly
LLM errors, lazy-loaded pdf-parse, and extended Nginx timeout.

frontend/src/App.tsx

@@ -1,6 +1,5 @@
 import { Routes, Route, Navigate } from 'react-router-dom';
 import { useAuth } from './context/AuthContext';
-import { ImportProvider } from './context/ImportContext';
 import { Layout } from './components/Layout';
 import { LoginPage } from './pages/LoginPage';
 import { HistoryPage } from './pages/HistoryPage';
@@ -19,16 +18,14 @@ export function App() {
   }
 
   return (
-    <ImportProvider>
-      <Layout>
-        <Routes>
-          <Route path="/" element={<Navigate to="/history" replace />} />
-          <Route path="/history" element={<HistoryPage />} />
-          <Route path="/analytics" element={<AnalyticsPage />} />
-          <Route path="/settings" element={<SettingsPage />} />
-          <Route path="*" element={<Navigate to="/history" replace />} />
-        </Routes>
-      </Layout>
-    </ImportProvider>
+    <Layout>
+      <Routes>
+        <Route path="/" element={<Navigate to="/history" replace />} />
+        <Route path="/history" element={<HistoryPage />} />
+        <Route path="/analytics" element={<AnalyticsPage />} />
+        <Route path="/settings" element={<SettingsPage />} />
+        <Route path="*" element={<Navigate to="/history" replace />} />
+      </Routes>
+    </Layout>
   );
 }