Messages: post as an organization (organizationId) #39

POST /api/messages accepts organizationId, letting a post be attributed to an organization the user belongs to. The app's composer has no such selector.

Acceptance criteria

• PostMessageAsync accepts an optional organizationId.
• Composer gains a "post as" selector listing the user's organizations (GET /api/user/organizations, already implemented) plus themselves.
• Feed cards attribute an organization post to the organization.
• Verify the write live on the test account before typing the response strictly (read-after-write).