feat(todo): comments on a task (ADR 0033, D36)

Plain text with links, written, edited and deleted in the task's sheet,
counted on its row. They are sealed in `task_comment`, the task they belong
to with them, and go with their task.

Whats-New: Comments: write on a task, edit or delete what you wrote, and open its links.
Whats-New-De: Kommentare: Schreibe zu einer Aufgabe, bearbeite oder lösche es und öffne Links darin.

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/pubspec.yaml

@@ -61,6 +61,7 @@ dependencies:
   # The instant a reminder is scheduled at, as flutter_local_notifications
   # takes it (in UTC).
   timezone: ^0.11.2
+  url_launcher: ^6.3.2
   # Pinned as Train's: the 9 line is beta, the only one for flutter_map 8.
   vector_map_tiles: 9.0.0-beta.13