Add gallery loading indicators and recover from WebGL context loss.

- Show loading markers while the catalog, portrait thumbnails, and 3D
  halls load so users know loading is still in progress.
- Recover the 3D hall from a lost WebGL context by remounting the canvas
  with a fresh context instead of leaving a permanent dark window, and
  guard the HDR environment map behind an error boundary.
- Show movement streams whose span overlaps the view even when their
  artists lived outside the visible time range.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/components/GalleryLoadingMarker.tsx

@@ -0,0 +1,35 @@
+import './GalleryLoadingMarker.css';
+
+interface Props {
+  message?: string;
+  /** Full-area overlay (dims background). */
+  overlay?: boolean;
+  /** Compact strip along the bottom — does not block interaction. */
+  banner?: boolean;
+  className?: string;
+}
+
+export default function GalleryLoadingMarker({
+  message = 'Loading…',
+  overlay = false,
+  banner = false,
+  className = '',
+}: Props) {
+  const modeClass = overlay
+    ? ' gallery-loading-marker-overlay'
+    : banner
+      ? ' gallery-loading-marker-banner'
+      : '';
+
+  return (
+    <div
+      className={`gallery-loading-marker${modeClass}${className ? ` ${className}` : ''}`}
+      role="status"
+      aria-live="polite"
+      aria-busy="true"
+    >
+      <div className="gallery-loading-marker-spinner" aria-hidden />
+      <p>{message}</p>
+    </div>
+  );
+}