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>

README.md

@@ -96,7 +96,8 @@ silently stay blank instead of erroring.
 
 **Library browsing**
 - Lazy-loading folder browser (grid or list view) with search, including an
-  opt-in transcript search across subtitle files
+  opt-in transcript search across subtitle files. Sortable by name,
+  progress, or how recently a course showed up in the library
 - Course cards show media file count, total video/audio runtime, and
   completion % at a glance
 - Cover art: uses a manually-placed `cover`/`folder`/`thumbnail`/`thumb`/