Add progress/recently-added sort to Library browser

The sort dropdown only had Name (A-Z/Z-A) - no way to surface "what
did I just download" or "what am I closest to finishing" without
scrolling. Adds Progress and Recently Added, backed by each course's
completion_percentage and folder mtime (already/newly attached in
list_library_directory and search_library_courses). Folders have
neither, so they naturally sort to the end under these two modes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

offlineu_core.py

@@ -1288,6 +1288,10 @@ def list_library_directory(dir_path: str, skip_hidden: bool = True) -> Dict[str,
             item['hidden'] = is_hidden
             item['favorited'] = os.path.abspath(entry_path_str) in favorite_set
             item['completion_percentage'] = _course_completion_percentage(entry, item['media_files'])
+            try:
+                item['mtime'] = entry.stat().st_mtime
+            except OSError:
+                item['mtime'] = 0
             items.append(item)
         else:
             if skip_hidden:
@@ -2078,6 +2082,10 @@ def search_library_courses(query: str) -> List[Dict[str, Any]]:
         item = _course_summary(course)
         item['favorited'] = os.path.abspath(str(course)) in favorite_set
         item['completion_percentage'] = _course_completion_percentage(course, item['media_files'])
+        try:
+            item['mtime'] = course.stat().st_mtime
+        except OSError:
+            item['mtime'] = 0
         results.append(item)
     return results