feat(v2): load older messages in pod chat

[lilyshen0722]

Reported: "the load older messages thing works on the public-read showcase room but it doesn't work on a real entered pod."

It was never built there

Not a broken feature. #764 added paging to the showcase reader only. The authenticated path fired exactly one request and kept no cursor:

// useV2PodDetail.ts — the complete message-read implementation, before
api.get(`/api/messages/${id}?limit=50`)   // no before, no hasMore, no loadOlder

V2PodChat had no button, no scroll handler, no observer. So any conversation past 50 messages was simply unreachable in a pod you had joined.

The backend already supported it — messageController accepts before, and both readers bottom out in the same PGMessage.findByPodId. The whole gap was client-side.

What's here

• useV2PodDetail gains hasMore / loadingOlder / loadOlder. End-of-history is inferred from a short page, because this endpoint returns a bare array and adding an envelope would break every existing consumer.
• Pages merge by id, not concat — the socket can deliver a message mid-flight and a concat would duplicate it.
• The pager renders at the top of the scroll container, styled to match the showcase pill so both reading surfaces agree.
• i18n keys in en and zh-CN.

Two scroll defects that would have made it look broken

1. The auto-scroll effect keyed on messages.length, so prepending a page threw the reader from the older message they'd just asked for straight back to the bottom. Now keyed on the newest message's id, so prepends are ignored.
2. A prepend changes scrollHeight and jumps the viewport. The reader's position is held by restoring distance-from-bottom, which is invariant under a prepend.

Neither is visible to jsdom, which is why they're called out rather than assumed fixed.

Test

6 hook tests: cursor correctness, prepend order, dedupe against an overlapping socket message, no-op on an empty pod, and hasMore surviving a failed request so the button stays retryable.

Frontend suite 69/69 suites, 316 tests. Lint identical to baseline (163 problems, the 1 error pre-existing in PersonalityBuilder.tsx). Typecheck: one pre-existing error remains, one removed.

Wants a real-browser check after deploy — per CLAUDE.md, jsdom has no layout engine, and the scroll-anchor behaviour above is exactly the class of thing it can't cover.

🤖 Generated with Claude Code