feat(todo): a project linked to a calendar shows its events beside the tasks (ADR 0033, D39)

A project's editor takes a calendar's address (https:// or webcal://,
refused otherwise), and its events show read-only, marked in the
project's colour: in Today's list and Upcoming's agenda above each day's
tasks, as blocks in the hour grids (Today's Calendar, Upcoming's 3-day)
sharing the lanes with timed tasks and all-day ones under All day, in
the project's Calendar (its agenda and month, its own events alone), and
in saved filters of days. A tap shows what an event is: when, where,
and whose calendar. The grid's lane layout is generic for it
(`layOutItems`); a task's row's box is shared (`rowBox`).

Whats-New: Link a calendar to a project in its settings: its events show beside your tasks in Today, Upcoming and the project.
Whats-New-De: Verknüpfe einen Kalender in den Einstellungen eines Projekts: Seine Termine erscheinen neben deinen Aufgaben in Heute, Demnächst und im Projekt.

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/task/day_grid_test.dart

@@ -20,7 +20,7 @@ void main() {
     occurrence: false,
   );
 
-  String laid(List<GridBlock> blocks) => [
+  String laid(List<GridBlock<CalendarEntry>> blocks) => [
     for (final b in blocks)
       '${b.entry.task.id} ${b.start}-${b.end} ${b.lane}/${b.lanes}',
   ].join('; ');