feat(todo): the open sidebar lists favourites and the project tree, folds kept

Todo's open sidebar is laid out as Todoist's (the developer's decisions
of 10 Nov 2026): the favourite projects, filters and labels, then the
projects as a tree with their open counts, sub-projects folding. Browse
and the sidebar share their rows (project_lists.dart) and their folds,
which the device now keeps per person (FoldedProjects,
folded_projects.json in the person's folder) instead of forgetting them
at each start.

Whats-New: On a wide screen, the open sidebar lists your favourites and projects; folded sub-projects stay folded.
Whats-New-De: Auf breiten Bildschirmen zeigt die geöffnete Seitenleiste deine Favoriten und Projekte; eingeklappte Unterprojekte bleiben eingeklappt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/feature/project/folded_projects_test.dart

@@ -0,0 +1,51 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/data/folded_projects.dart';
+
+/// Which projects' sub-projects are folded away, kept per device (the
+/// developer's decision of 10 Nov 2026), in Browse and the sidebar alike.
+void main() {
+  late Directory folder;
+  late File file;
+
+  setUp(() {
+    folder = Directory.systemTemp.createTempSync('folded');
+    file = File('${folder.path}/folded_projects.json');
+  });
+  tearDown(() => folder.deleteSync(recursive: true));
+
+  test('nothing is folded at first', () async {
+    final folded = await FoldedProjects.open(file);
+
+    expect(folded.isFolded('school'), isFalse);
+  });
+
+  test('a fold is kept for the next start, an unfold too', () async {
+    final folded = await FoldedProjects.open(file);
+    var told = 0;
+    folded.addListener(() => told++);
+
+    await folded.toggle('school');
+    await folded.toggle('home');
+    await folded.toggle('home');
+
+    expect(told, 3);
+    final again = await FoldedProjects.open(file);
+    expect(again.isFolded('school'), isTrue);
+    expect(again.isFolded('home'), isFalse);
+  });
+
+  test('a damaged file folds nothing', () async {
+    file.writeAsStringSync('{"school": ');
+
+    expect((await FoldedProjects.open(file)).isFolded('school'), isFalse);
+  });
+
+  test('without a file it folds for this session alone', () async {
+    final folded = FoldedProjects();
+    await folded.toggle('school');
+
+    expect(folded.isFolded('school'), isTrue);
+  });
+}