feat: bookmark ("save for later") icon on posts

[arturasmckwcz]

Context

Bookmark UI for the Posts page rebuild, per the prototype proposal. Blocked on need4deed-org/be#929 (Bookmark entity + endpoint + SDK types) — persistence needs to be server-side/per-user so it survives across devices and sessions, not just localStorage.

Scope

• Icon-only bookmark toggle on each post card
• Persists via the new bookmark endpoint

Acceptance criteria

• Bookmark icon toggles saved state for the current user
• State persists across reloads/devices (server-backed, not local-storage-only)

Blocked on: need4deed-org/be#929
Part of Epic: #969