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/lib/l10n/app_de.arb

@@ -404,5 +404,10 @@
   "locationArrived": "Du bist bei {name}",
   "locationLeft": "Du hast {name} verlassen",
   "tasksDeleteCompleted": "Erledigte Aufgaben löschen",
-  "tasksCompletedDeleted": "{count, plural, =1{1 erledigte Aufgabe gelöscht} other{{count} erledigte Aufgaben gelöscht}}"
+  "tasksCompletedDeleted": "{count, plural, =1{1 erledigte Aufgabe gelöscht} other{{count} erledigte Aufgaben gelöscht}}",
+  "projectCalendarTitle": "Verknüpfter Kalender",
+  "projectCalendarAddress": "Kalenderadresse (iCal)",
+  "projectCalendarHelp": "Seine Termine stehen neben den Aufgaben dieses Projekts, nur zum Lesen. Jedes deiner Geräte ruft ihn selbst ab.",
+  "projectCalendarInvalid": "Eine Adresse, die mit https:// oder webcal:// beginnt",
+  "eventFromCalendar": "Aus dem Kalender von {project}"
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_en.arb

@@ -1262,5 +1262,30 @@
         "type": "int"
       }
     }
+  },
+  "projectCalendarTitle": "Linked calendar",
+  "@projectCalendarTitle": {
+    "description": "The project's editor: the calendar linked to it, whose events show beside its tasks (ADR 0033, D39)."
+  },
+  "projectCalendarAddress": "Calendar address (iCal)",
+  "@projectCalendarAddress": {
+    "description": "The project's editor: the field for the calendar's feed address (ADR 0033, D39)."
+  },
+  "projectCalendarHelp": "Its events show beside this project's tasks, read-only. Each of your devices fetches it itself.",
+  "@projectCalendarHelp": {
+    "description": "The project's editor, below the calendar's address (ADR 0033, D39)."
+  },
+  "projectCalendarInvalid": "An address starting with https:// or webcal://",
+  "@projectCalendarInvalid": {
+    "description": "The project's editor: an address no feed can be fetched from (ADR 0033, D39)."
+  },
+  "eventFromCalendar": "From {project}'s calendar",
+  "@eventFromCalendar": {
+    "description": "An event's details: which project's calendar it comes from (ADR 0033, D39).",
+    "placeholders": {
+      "project": {
+        "type": "String"
+      }
+    }
   }
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations.dart

@@ -2529,6 +2529,36 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'{count, plural, =1{1 completed task deleted} other{{count} completed tasks deleted}}'**
   String tasksCompletedDeleted(int count);
+
+  /// The project's editor: the calendar linked to it, whose events show beside its tasks (ADR 0033, D39).
+  ///
+  /// In en, this message translates to:
+  /// **'Linked calendar'**
+  String get projectCalendarTitle;
+
+  /// The project's editor: the field for the calendar's feed address (ADR 0033, D39).
+  ///
+  /// In en, this message translates to:
+  /// **'Calendar address (iCal)'**
+  String get projectCalendarAddress;
+
+  /// The project's editor, below the calendar's address (ADR 0033, D39).
+  ///
+  /// In en, this message translates to:
+  /// **'Its events show beside this project\'s tasks, read-only. Each of your devices fetches it itself.'**
+  String get projectCalendarHelp;
+
+  /// The project's editor: an address no feed can be fetched from (ADR 0033, D39).
+  ///
+  /// In en, this message translates to:
+  /// **'An address starting with https:// or webcal://'**
+  String get projectCalendarInvalid;
+
+  /// An event's details: which project's calendar it comes from (ADR 0033, D39).
+  ///
+  /// In en, this message translates to:
+  /// **'From {project}\'s calendar'**
+  String eventFromCalendar(String project);
 }
 
 class _AppLocalizationsDelegate

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_de.dart

@@ -1589,4 +1589,23 @@ class AppLocalizationsDe extends AppLocalizations {
     );
     return '$_temp0';
   }
+
+  @override
+  String get projectCalendarTitle => 'Verknüpfter Kalender';
+
+  @override
+  String get projectCalendarAddress => 'Kalenderadresse (iCal)';
+
+  @override
+  String get projectCalendarHelp =>
+      'Seine Termine stehen neben den Aufgaben dieses Projekts, nur zum Lesen. Jedes deiner Geräte ruft ihn selbst ab.';
+
+  @override
+  String get projectCalendarInvalid =>
+      'Eine Adresse, die mit https:// oder webcal:// beginnt';
+
+  @override
+  String eventFromCalendar(String project) {
+    return 'Aus dem Kalender von $project';
+  }
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_en.dart

@@ -1586,6 +1586,25 @@ class AppLocalizationsEn extends AppLocalizations {
     );
     return '$_temp0';
   }
+
+  @override
+  String get projectCalendarTitle => 'Linked calendar';
+
+  @override
+  String get projectCalendarAddress => 'Calendar address (iCal)';
+
+  @override
+  String get projectCalendarHelp =>
+      'Its events show beside this project\'s tasks, read-only. Each of your devices fetches it itself.';
+
+  @override
+  String get projectCalendarInvalid =>
+      'An address starting with https:// or webcal://';
+
+  @override
+  String eventFromCalendar(String project) {
+    return 'From $project\'s calendar';
+  }
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).