Add anonymous post toggle to post composer

[JoshuaNam]

Description

Add a checkbox/toggle in the post creation UI that allows users to mark their post as semi-anonymous.

Acceptance Criteria

• A checkbox labeled "Post anonymously" appears below the post content textarea
• Checkbox is unchecked by default
• When user submits the post, the checkbox value (true/false) is included in the POST request body
• Checkbox is hidden if admin has disabled anonymous posting (see admin settings issue)

Community forum reference

No response