feat(feed/chat): lazy-render + pagination for long scrolls

Server pagination
  - blockchain.PostsByAuthor signature extended with beforeTs int64;
    passing 0 keeps the previous "everything, newest first" behaviour,
    non-zero skips posts with CreatedAt >= beforeTs so clients can
    paginate older results.
  - node.FeedConfig.PostsByAuthor callback type updated; the two
    /feed endpoints that use it (timeline + author) now accept
    `?before=<unix_seconds>` and forward it through. /feed/author
    limit default dropped from 50 to 30 to match the client's page
    size.
  - node/api_common.go: new queryInt64 helper for parsing the cursor
    param safely (matches the queryInt pattern already used).

Client infinite scroll (Feed tab)
  - lib/feed.ts: fetchTimeline / fetchAuthorPosts accept
    `{limit?, before?}` options. Old signatures still work for other
    callers (fetchForYou / fetchTrending / fetchHashtag) — those are
    ranked feeds that don't have a stable cursor so they stay
    single-shot.
  - feed/index.tsx: tracks loadingMore / exhausted state. onEndReached
    (threshold 0.6) fires loadMore() which fetches the next 20 posts
    using the oldest currently-loaded post's created_at as `before`.
    Deduplicates on post_id before appending. Stops when the server
    returns < PAGE_SIZE items. ListFooterComponent shows a small
    spinner during paginated fetches.
  - FlatList lazy-render tuning on all feed lists (index + hashtag):
    initialNumToRender:10, maxToRenderPerBatch:8, windowSize:7,
    removeClippedSubviews — first paint stays quick even with 100+
    posts loaded.

Chat lazy render
  - chats/[id].tsx FlatList: initialNumToRender:25 (~1.5 screens),
    maxToRenderPerBatch:12, windowSize:10, removeClippedSubviews.
    Keeps initial chat open snappy on conversations with thousands
    of messages; RN re-renders a small window around the viewport
    and drops the rest.

Tests
  - chain_test.go updated for new PostsByAuthor signature.
  - All 7 Go packages green.
  - tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

node/api_common.go

@@ -38,6 +38,20 @@ func queryInt(r *http.Request, key string, def int) int {
 	return n
 }
 
+// queryInt64 reads a non-negative int64 query param — typically a unix
+// timestamp cursor for pagination. Returns def when missing or invalid.
+func queryInt64(r *http.Request, key string, def int64) int64 {
+	s := r.URL.Query().Get(key)
+	if s == "" {
+		return def
+	}
+	n, err := strconv.ParseInt(s, 10, 64)
+	if err != nil || n < 0 {
+		return def
+	}
+	return n
+}
+
 // queryIntMin0 parses a query param as a non-negative integer; returns 0 if absent or invalid.
 func queryIntMin0(r *http.Request, key string) int {
 	s := r.URL.Query().Get(key)