Add holiday party photos to gallery

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: madan96

_data/gallery.yml

@@ -1,3 +1,9 @@
+- title: "Holiday Party"
+  caption: "The EmPRISE lab celebrated the holiday season together."
+  images:
+    - "social1.jpg"
+    - "social2.jpg"
+
 - title: "Going Out for Lab Lunch"
   caption: "Our lab took a break from research to enjoy a wonderful lunch together."
   images: