LinkedIn: link-as-first-comment option

[Adron]

POST /api/messages accepts linkedInLinkAsFirstComment — "post the link as the first comment instead of in the body. Only meaningful when the content contains a URL."

A well-known LinkedIn reach tactic, and a one-field change on a path the app already uses.

Acceptance criteria

• PostMessageAsync accepts the flag.
• A composer toggle that appears only when the draft contains a URL and LinkedIn is a selected destination.
• Also passed through by the AI Message Series crossPost object (AI epic).