User Story: Share Milestones

[lienebriede]

As a registered user, I want to share my milestone achievements on the feed so that others can see my progress.

Acceptance Criteria

• When achieving a milestone, users see a "Share to Feed" button.
• Clicking it posts the milestone message to the feed.

Tasks

• Add a "Share to Feed" button to milestone modal.
• Implement API call to post milestone messages.