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.css

@@ -0,0 +1,61 @@
+.gallery-loading-marker {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 14px;
+  color: rgba(232, 213, 181, 0.92);
+  font-family: Georgia, serif;
+  font-size: 15px;
+  text-align: center;
+  pointer-events: none;
+}
+
+.gallery-loading-marker p {
+  margin: 0;
+}
+
+.gallery-loading-marker-spinner {
+  width: 36px;
+  height: 36px;
+  border-radius: 50%;
+  border: 3px solid rgba(201, 169, 110, 0.22);
+  border-top-color: #c9a96e;
+  animation: gallery-loading-spin 0.85s linear infinite;
+}
+
+.gallery-loading-marker-overlay {
+  position: absolute;
+  inset: 0;
+  z-index: 20;
+  background: rgba(10, 10, 20, 0.72);
+  backdrop-filter: blur(2px);
+}
+
+.gallery-loading-marker-banner {
+  position: absolute;
+  left: 50%;
+  bottom: 12px;
+  transform: translateX(-50%);
+  z-index: 12;
+  flex-direction: row;
+  gap: 10px;
+  padding: 8px 16px;
+  border-radius: 999px;
+  background: rgba(15, 15, 26, 0.88);
+  border: 1px solid rgba(201, 169, 110, 0.35);
+  font-size: 13px;
+  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.35);
+}
+
+.gallery-loading-marker-banner .gallery-loading-marker-spinner {
+  width: 18px;
+  height: 18px;
+  border-width: 2px;
+}
+
+@keyframes gallery-loading-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}