fix: alarm comment crash and wrong display format

[deaflynx]

Summary

Fixes a crash when viewing alarm comments and a display issue where system activity entries (assignments, status changes) were rendered as raw JSON {text: ..., subtype: ..., userName: ...} instead of the actual message.

Root cause

The ThingsBoard API expects the alarm comment payload as a JSON object ({"text": "..."}), but the mobile app was posting comments as plain String values. The server tolerates both formats on write, but returns whatever was stored:

• System-generated and web-posted comments → returned as Map
• Mobile-posted comments (legacy) → returned as String

AlarmComment.comment is typed as dynamic in the client library, so widgets calling .edited / accessing the full text assumed a typed object and crashed with NoSuchMethodError: Class 'String' has no instance getter 'edited' whenever they encountered a legacy mobile comment. System activity widgets called toString() on the map, producing the raw {text: ..., subtype: ...} output.

Changes

Flutter app (flutter_thingsboard_app)

• alarm_details_datasource.dart — post/update now send the API-expected {'text': comment} payload instead of a raw string. Removed _parseComment workaround since parsing is now handled in the client library.
• Activity widgets (user_comment_widget.dart, system_activity_widget.dart, alarm_edit_comment_textfield.dart) — use (comment as AlarmCommentJsonNode).text / .edited instead of dynamic access.

Dart client library (thingsboard_client)

• See companion PR: fix: parse AlarmComment.comment into AlarmCommentJsonNode on deserialization dart_thingsboard_client#61
• AlarmComment.fromJson — parses comment field into AlarmCommentJsonNode automatically (handles both Map and legacy String formats).
• AlarmComment.toJson — serializes AlarmCommentJsonNode back to a Map for the API.

Note: This PR depends on thingsboard/dart_thingsboard_client#61. Once that client library change is published, the datasource in this PR is a clean pass-through.

Test plan

• Post a new comment on an alarm — renders correctly, no crash
• Edit an existing comment — edit dialog pre-populates with the text, "edited" label appears after save
• View an alarm that has a legacy mobile-posted comment (stored as raw string) — renders correctly, no crash
• View an alarm with system activity (assignment/unassignment, status change) — shows the human-readable message, not the raw JSON