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_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."