Include `seen` status in `hey box` JSON output

[arnthorsnaer]

Currently, the raw HEY API (https://app.hey.com/imbox.json) returns a seen field for each posting:

{
  "id": 1004013038,
  "name": "Subject line",
  "seen": false
}

However, hey box <name> --json does not include this field in its output, making it impossible to filter for unread emails without directly calling the API with curl.

Use case:

• Filter for unread emails in a specific box
• Build automation that only processes unseen emails
• Replicate the TUI's read/unread functionality in scripts

Current workaround:

curl -H "Authorization: Bearer $(hey auth token --quiet)" \
  "https://app.hey.com/imbox.json" | \
  jq '.postings[] | select(.seen == false)'

Request:
Include the seen field in hey box JSON output to match the raw API response.