persist font size, add :fav/:sub/:content commands

- note_editor: persist font size to SharedPreferences on change and
  restore it on load
- note_list: add :fav (toggle favorites filter), :sub (toggle
  subdirectory search), :content (toggle file content search) commands
- texts: document new commands in help content

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

lib/note_list.dart

@@ -669,6 +669,16 @@ Happy note taking! ✨
       case ':intern':
         _toggleExternalEditor(false);
         break;
+      case ':fav':
+        setState(() { _showOnlyFavorites = !_showOnlyFavorites; });
+        _filterNotes();
+        break;
+      case ':sub':
+        _toggleIncludeSubdirectories(!_includeSubdirectories);
+        break;
+      case ':content':
+        _toggleIncludeFileContent(!_includeFileContent);
+        break;
       case ':emptytrash':
         _confirmEmptyTrash();
         break;