fix: parse AlarmComment.comment into AlarmCommentJsonNode on deserialization

[deaflynx]

Summary

AlarmComment.fromJson now automatically parses the dynamic comment field into a typed AlarmCommentJsonNode, handling both Map (normal) and String (legacy mobile-posted) formats. AlarmComment.toJson properly serializes it back for the API.

Root cause

The mobile app was posting comments as raw strings instead of {"text": "..."} objects. The server stores whatever it receives, so on read the comment field could be either a Map or a String. Since comment is typed as dynamic, any consumer accessing .text or .edited would crash with NoSuchMethodError on the wrong type.

Changes

• AlarmComment.fromJson — calls _parseComment() to normalize comment into AlarmCommentJsonNode
• AlarmComment.toJson — serializes AlarmCommentJsonNode back to a Map via .toJson()
• _parseComment static method — handles Map → AlarmCommentJsonNode.fromJson(), String → wraps in AlarmCommentJsonNode(text: ...), other → pass-through

Test plan

• Deserialize an alarm comment with comment as a JSON object ({"text": "hello"}) — produces AlarmCommentJsonNode
• Deserialize an alarm comment with comment as a raw string ("hello") — produces AlarmCommentJsonNode with text: "hello"
• Serialize an AlarmComment with AlarmCommentJsonNode comment — produces correct JSON map
• Post and update alarm comments via AlarmService — round-trips correctly