feat: Post card — author/tags/opportunity chips, three-dot edit/delete menu with confirm dialog

[arturasmckwcz]

Context

Post display + management for the Posts page rebuild. Not blocked on new backend work — uses the existing PATCH/DELETE /post/:id endpoints (be#680).

Scope

• Post card: author name/avatar, timestamp, text with tag rendering (reuse the same rendering used for Comments), linked-opportunity chips that link to the opportunity's profile
• Three-dot menu on a post: Edit / Delete (author-only, or admin/coordinator per existing role conventions)
• Confirmation pop-up before delete
• Removes the old placeholder's hardcoded-disabled Send button and no-op textarea (final cleanup of the feat: Posts page — NGO/Coordinator feed with posting, @mention tagging, and linking own opportunities #892 placeholder)

Acceptance criteria

• Post card shows author, timestamp, tagged users, and clickable opportunity chips
• Three-dot menu offers Edit and Delete to the post's author
• Deleting a post requires confirmation first
• Old placeholder composer/button code is removed

Part of Epic: #969