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>

templates/course_dashboard.html

@@ -1243,6 +1243,8 @@
                     <select id="library-sort" class="library-sort-select" onchange="reapplySort()">
                         <option value="name-asc">Name (A→Z)</option>
                         <option value="name-desc">Name (Z→A)</option>
+                        <option value="progress">Progress</option>
+                        <option value="recently-added">Recently Added</option>
                     </select>
                     <div class="library-view-toggle">
                         <button type="button" class="view-toggle-btn" id="library-view-list-btn"
@@ -1889,9 +1891,19 @@
         function sortLibraryItems(items) {
             const order = document.getElementById('library-sort').value;
             const sorted = items.slice();
-            sorted.sort((a, b) => order === 'name-desc'
-                ? b.name.localeCompare(a.name)
-                : a.name.localeCompare(b.name));
+            // Directories have neither a completion_percentage nor an
+            // mtime (only courses do), so they naturally fall to the end
+            // under these two modes rather than needing separate grouping
+            // logic - falling back to name order among themselves/ties.
+            if (order === 'progress') {
+                sorted.sort((a, b) => (b.completion_percentage || 0) - (a.completion_percentage || 0) || a.name.localeCompare(b.name));
+            } else if (order === 'recently-added') {
+                sorted.sort((a, b) => (b.mtime || 0) - (a.mtime || 0) || a.name.localeCompare(b.name));
+            } else {
+                sorted.sort((a, b) => order === 'name-desc'
+                    ? b.name.localeCompare(a.name)
+                    : a.name.localeCompare(b.name));
+            }
             return sorted;
         }