feat(todo): a project's editor as Todoist's Add project, with sub-projects and favourites

Adding or editing a project was a dialog of a name and a colour. It is
now a screen of its own on a phone (a dialog where sheets are dialogs),
as Todoist's Add project: the name counted to 120, a description, the
colour, the parent project, a Favourite switch and the layout, Display's
pictures. All of it lives in the project's sealed payload, the schema
still 1, no server model or migration; the payload now keeps fields it
doesn't know, as a task's does.

A new project starts with the layout picked, and Display's Reset all
returns to that default rather than to List. Browse lists sub-projects
indented and folding under their parents, and the favourites in a group
above; a project is never offered as its own ancestor, and a parent's
deletion takes its sub-projects, naming them first, as Todoist's. ADR
0033, D28.

Whats-New: Add project and Edit project are a screen of their own, with a description, a parent project, Favourite and the layout.
Whats-New: Browse lists sub-projects under their parents and your favourite projects at the top.
Whats-New: Reset all in Display returns to the layout the project was made with.
Whats-New-De: „Projekt hinzufügen“ und „Projekt bearbeiten“ sind eine eigene Seite, mit Beschreibung, übergeordnetem Projekt, Favorit und Layout.
Whats-New-De: „Übersicht“ zeigt Unterprojekte unter ihren Projekten und deine Favoriten oben.
Whats-New-De: „Alles zurücksetzen“ in der Anzeige kehrt zum Layout zurück, mit dem das Projekt angelegt wurde.

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

@@ -106,7 +106,7 @@
   "priority4": "No priority",
   "projectsTitle": "Projects",
   "projectsNone": "No projects of your own yet.",
-  "projectAdd": "New project",
+  "projectAdd": "Add project",
   "projectEdit": "Edit project",
   "projectDelete": "Delete project",
   "projectDeleteTitle": "Delete the project?",
@@ -118,6 +118,27 @@
       }
     }
   },
+  "projectParent": "Parent project",
+  "@projectParent": {
+    "description": "A project's editor (ADR 0033, D28), as Todoist's Add project."
+  },
+  "projectNoParent": "No parent",
+  "projectFavorite": "Favourite",
+  "favoritesTitle": "Favourites",
+  "projectSubProjectsHide": "Hide sub-projects",
+  "projectSubProjectsShow": "Show sub-projects",
+  "projectDeleteWithSubProjects": "\"{name}\" is deleted with its sub-projects {subProjects}, and their sections and tasks, on all your devices.",
+  "@projectDeleteWithSubProjects": {
+    "description": "Deleting a project with sub-projects (ADR 0033, D28): subProjects their names, separated by commas.",
+    "placeholders": {
+      "name": {
+        "type": "String"
+      },
+      "subProjects": {
+        "type": "String"
+      }
+    }
+  },
   "projectEmptyTitle": "Nothing to do here",
   "projectEmptyHint": "Add a task with the + button.",
   "projectGone": "This project was deleted.",