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_editor.dart

@@ -56,6 +56,7 @@ class _NoteEditorScreenState extends State<NoteEditorScreen> {
     final prefs = await SharedPreferences.getInstance();
     setState(() {
       _autoSaveEnabled = prefs.getBool('auto_save_enabled') ?? false;
+      _fontSize = prefs.getDouble('font_size') ?? 16.0;
     });
   }
 
@@ -164,6 +165,7 @@ class _NoteEditorScreenState extends State<NoteEditorScreen> {
     setState(() {
       _fontSize = newSize;
     });
+    SharedPreferences.getInstance().then((prefs) => prefs.setDouble('font_size', newSize));
   }
 
   @override