feat: emoji reactions on posts and replies (quick bar + searchable picker)

[arturasmckwcz]

Context

Reactions UI for the Posts page rebuild, per the prototype proposal. Blocked on need4deed-org/be#928 (Reaction entity + endpoints + SDK types).

Scope

• Quick-reaction bar with a handful of common emoji, plus a full searchable emoji picker for anything else
• Available on both posts and replies
• Shows aggregated per-emoji counts and highlights the current user's own reaction

Acceptance criteria

• Users can react to a post or reply from the quick bar or the full picker
• Reaction counts update and reflect the current user's own reaction state
• Works consistently on posts and on replies

Blocked on: need4deed-org/be#928
Part of Epic: #969