feat: Posts feed — chat-direction ordering (oldest top, newest bottom) + pagination

[arturasmckwcz]

Context

Feed layout for the Posts page rebuild. The amendment reversed the original proposal's ordering: posts now read like a chat, oldest at the top, newest at the bottom, with the composer pinned below the feed (see fe#961 for the composer itself). This also reverses #892's original "newest first" acceptance criterion — that's superseded.

Scope

• Feed renders oldest → newest top-to-bottom, auto-scrolled to the bottom (newest) on load, matching chat UX
• "Load older" fetches the previous page and prepends it at the top (reverse-infinite-scroll), replacing feat: Posts page — NGO/Coordinator feed with posting, @mention tagging, and linking own opportunities #892's original "More posts" button-at-bottom pattern
• Uses existing GET /post pagination — no BE change needed

Acceptance criteria

• Feed opens scrolled to the newest post at the bottom
• Scrolling up near the top loads and prepends older posts
• No layout jump when older posts are prepended

Part of Epic: #969