feat(todo): the Plan beside Upcoming, with what has no date (ADR 0033, D34)

Upcoming's 3-day grid gets the Plan Today's calendar has, through what
the two tabs now share (PlanHost). Its new last group holds the open
tasks of no day, so a week can be planned from the Inbox by dropping them
on an hour; they aren't counted on the button, which says what is due.

Whats-New: Upcoming's 3-day view has the Plan too, and tasks without a date can be dragged from it onto any day.
Whats-New-De: Auch die 3-Tage-Ansicht unter Demnächst hat den Plan, und Aufgaben ohne Datum lassen sich von dort auf jeden Tag ziehen.

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

@@ -266,6 +266,7 @@
   "planTitle": "Plan",
   "planShow": "Plan einblenden",
   "planHide": "Plan ausblenden",
+  "planNoDate": "Ohne Datum",
   "planToPlan": "Zu planen",
   "planTimeBlocked": "Eingeplant",
   "planEmpty": "Nichts zu planen",

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

@@ -861,6 +861,7 @@
   "@planHide": {
     "description": "Tooltip of Today's Plan button and the Plan's close button while it shows (ADR 0033, D30)."
   },
+  "planNoDate": "No date",
   "planToPlan": "To plan",
   "@planToPlan": {
     "description": "Today's Plan (ADR 0033, D30): the group of today's tasks without a time, to be given one."

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

@@ -1696,6 +1696,12 @@ abstract class AppLocalizations {
   /// **'Hide the Plan'**
   String get planHide;
 
+  /// No description provided for @planNoDate.
+  ///
+  /// In en, this message translates to:
+  /// **'No date'**
+  String get planNoDate;
+
   /// Today's Plan (ADR 0033, D30): the group of today's tasks without a time, to be given one.
   ///
   /// In en, this message translates to:

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

@@ -1047,6 +1047,9 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get planHide => 'Plan ausblenden';
 
+  @override
+  String get planNoDate => 'Ohne Datum';
+
   @override
   String get planToPlan => 'Zu planen';
 

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

@@ -1047,6 +1047,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get planHide => 'Hide the Plan';
 
+  @override
+  String get planNoDate => 'No date';
+
   @override
   String get planToPlan => 'To plan';