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/lib/l10n/app_en.arb

@@ -115,6 +115,23 @@
   "@filterNotReadEnd": {
     "description": "The filter editor when its query stops before it is complete."
   },
+  "commentsTitle": "Comments",
+  "commentAdd": "Add a comment",
+  "commentSend": "Send comment",
+  "commentMenu": "Comment menu",
+  "commentEdit": "Edit comment",
+  "commentCopy": "Copy text",
+  "commentDelete": "Delete comment",
+  "commentDeleteTitle": "Delete the comment?",
+  "commentEdited": "{when}, edited",
+  "@commentEdited": {
+    "description": "When a comment was written, once it was changed.",
+    "placeholders": {
+      "when": {
+        "type": "String"
+      }
+    }
+  },
   "labelsTitle": "Labels",
   "labelsNone": "No labels yet.",
   "labelsNoneSet": "No labels",