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/styles/index.css

@@ -1058,152 +1058,6 @@ textarea {
   font-weight: 500;
 }
 
-/* Import progress bar in modal */
-.import-progress-modal {
-  padding: 20px 0;
-}
-
-.import-progress-modal-bar {
-  height: 8px;
-  background: var(--color-border);
-  border-radius: 4px;
-  overflow: hidden;
-  margin-bottom: 10px;
-}
-
-.import-progress-modal-fill {
-  height: 100%;
-  background: linear-gradient(90deg, var(--color-primary), #6366f1);
-  border-radius: 4px;
-  transition: width 0.3s ease;
-  position: relative;
-}
-
-.import-progress-modal-fill::after {
-  content: '';
-  position: absolute;
-  inset: 0;
-  background: linear-gradient(
-    90deg,
-    transparent 0%,
-    rgba(255, 255, 255, 0.3) 50%,
-    transparent 100%
-  );
-  animation: shimmer 1.5s infinite;
-}
-
-.import-progress-modal-label {
-  text-align: center;
-  color: var(--color-text-secondary);
-  font-size: 14px;
-}
-
-/* ================================================================
-   Import progress bar (fixed top, Layout)
-   ================================================================ */
-
-.import-progress-bar {
-  position: fixed;
-  top: 0;
-  left: 0;
-  right: 0;
-  z-index: 300;
-  height: 4px;
-  background: var(--color-border);
-}
-
-.import-progress-bar--done {
-  background: var(--color-success-light);
-}
-
-.import-progress-bar--error {
-  background: var(--color-danger-light);
-}
-
-.import-progress-bar__fill {
-  height: 100%;
-  border-radius: 0 2px 2px 0;
-  transition: width 0.3s ease;
-  position: relative;
-  background: linear-gradient(90deg, var(--color-primary), #6366f1);
-}
-
-.import-progress-bar__fill::after {
-  content: '';
-  position: absolute;
-  inset: 0;
-  background: linear-gradient(
-    90deg,
-    transparent 0%,
-    rgba(255, 255, 255, 0.4) 50%,
-    transparent 100%
-  );
-  animation: shimmer 1.5s infinite;
-}
-
-.import-progress-bar--done .import-progress-bar__fill {
-  background: var(--color-success);
-}
-
-.import-progress-bar--done .import-progress-bar__fill::after {
-  animation: none;
-}
-
-.import-progress-bar--error .import-progress-bar__fill {
-  background: var(--color-danger);
-  width: 100% !important;
-}
-
-.import-progress-bar--error .import-progress-bar__fill::after {
-  animation: none;
-}
-
-@keyframes shimmer {
-  0% {
-    transform: translateX(-100%);
-  }
-  100% {
-    transform: translateX(100%);
-  }
-}
-
-.import-progress-label {
-  position: fixed;
-  top: 6px;
-  right: 16px;
-  z-index: 301;
-  background: var(--color-surface);
-  border: 1px solid var(--color-border);
-  border-radius: 16px;
-  padding: 4px 14px;
-  font-size: 12px;
-  font-weight: 500;
-  color: var(--color-text-secondary);
-  box-shadow: var(--shadow-sm);
-  white-space: nowrap;
-  cursor: default;
-  font-family: inherit;
-}
-
-.import-progress-label--clickable {
-  cursor: pointer;
-}
-
-.import-progress-label--clickable:hover {
-  background: var(--color-bg);
-  border-color: var(--color-border-hover);
-}
-
-.import-progress-bar--done .import-progress-label {
-  color: var(--color-success);
-  border-color: var(--color-success);
-}
-
-.import-progress-bar--error .import-progress-label {
-  color: var(--color-danger);
-  border-color: var(--color-danger);
-}
-
 /* ================================================================
    Tabs
    ================================================================ */