fix: Improve sorting and normalization of study dates in user uploads

Co-authored-by: Copilot <copilot@github.com>

atlas/views.py

@@ -2280,14 +2280,22 @@ def user_uploads(
         tags, date = data[series][0]
         series_list.append((series, len(data[series]), tags, date))
 
-    series_list.sort(key=lambda l: l[-1])
+    series_list.sort(key=lambda l: l[-1] or "")
 
     # Group series by StudyInstanceUID for better visual grouping in the template
     grouped = {}
     for series_uid, count, tags, date in series_list:
         study_uid = tags.get("StudyInstanceUID") or tags.get("StudyID") or "unknown"
         study_desc = tags.get("StudyDescription", "")
-        study_date = tags.get("StudyDate", date)
+        # Normalise to a sortable string; DICOM StudyDate is "YYYYMMDD", fall
+        # back to the upload's created_date converted to the same format.
+        raw_study_date = tags.get("StudyDate")
+        if raw_study_date:
+            study_date = str(raw_study_date)
+        elif hasattr(date, "strftime"):
+            study_date = date.strftime("%Y%m%d")
+        else:
+            study_date = str(date)
         if study_uid not in grouped:
             grouped[study_uid] = {
                 "description": study_desc,