Normalise gallery card thumbnail height

Gallery cards previously inherited each thumbnail's native aspect
ratio, producing uneven card heights. Pin .sd-card-img-top to a
fixed 200px box with `object-fit: contain` on a white background so
images centre inside a uniform slot regardless of source dimensions.

Author: timtreis

docs/_static/css/custom.css

@@ -2,3 +2,15 @@
 div.cell_output table.dataframe {
     font-size: 0.8em;
 }
+
+/* Gallery cards: normalise thumbnail height so cards line up regardless
+   of the underlying image aspect ratio. Image is contained (not cropped)
+   on a white background, centred in the slot. */
+.sd-card-img-top {
+    height: 200px;
+    width: 100%;
+    object-fit: contain;
+    background: #ffffff;
+    padding: 8px;
+    box-sizing: border-box;
+}