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/pubspec.yaml

@@ -52,6 +52,8 @@ dependencies:
   # Noto Sans and the widgets every screen is built of.
   halkyon_ui:
     path: ../../../packages/halkyon_ui
+  # Calendars' feeds fetched by the device (ADR 0033, D39).
+  http: ^1.6.0
   # gen-l10n's output imports it.
   intl: ^0.20.3
   latlong2: ^0.10.1