feat(todo): the calendars linked to projects fetched by the device and kept for offline (ADR 0033, D39)

`CalendarCubit` follows the projects' calendar links: a calendar linked
is shown at once from the copy kept and fetched by the device over HTTPS
(`HttpCalendarFeeds`, `webcal://` as HTTPS), as the app opens, every hour
while it is open and when a list is pulled down; a fetch that fails, or
answers with something that is no calendar, keeps the last copy; a link
taken off, or its project gone, takes its events and its copy along. The
copies are files in the person's own folder (`calendars/<project>.ics`).
Nothing shows the events yet.

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/support/fake_calendar_feeds.dart

@@ -0,0 +1,14 @@
+import 'package:halkyon_todo_flutter/src/feature/calendar/data/calendar_feeds.dart';
+
+/// Serves the calendar files [served] by their address, as a feed's host
+/// would, and remembers each address fetched; one it doesn't serve fails.
+class FakeCalendarFeeds() implements CalendarFeedGateway {
+  final served = <Uri, String>{};
+  final fetched = <Uri>[];
+
+  @override
+  Future<String> fetch(Uri url) async {
+    fetched.add(url);
+    return served[url] ?? (throw CalendarFeedFailed('$url not served'));
+  }
+}