ux(showcase): smooth one-at-a-time cadence + earlier infinite-scroll trigger

Operator-flagged 2026-05-30 (round 2): the batched-loads change improved
consistency but still felt "chunky" — 5 items appeared together, then a
~200 ms API round-trip pause, then another 5. And infinite-scroll fired
too late when a single tall image (long manga page) made one masonry
column much taller than its siblings.

**Cadence (showcase store):**
- Fire all INITIAL_BATCHES fetches in PARALLEL — collapses the per-
  batch round-trip gap so all the data arrives in ~1 RTT instead of 12
  sequential RTTs.
- Trickle each response's items into images.value one at a time with
  APPEND_DELAY_MS = 80ms between each (≈ the MasonryGrid stagger
  animation, 70ms). User sees a smooth steady stream.
- fetchPage (the infinite-scroll path) uses the same trickle so its
  5-item appends also cascade one-by-one instead of popping together.
- Sequence token guards against a fast shuffle / mount-then-shuffle
  interleaving two trickles into the same images.value.
- Dropped useAsyncAction here — the parallel-fetch-then-trickle flow
  doesn't fit its single-wrap-call shape cleanly; inline loading/error
  state is clearer.

**Infinite-scroll trigger (MasonryGrid):**
- Pass `rootMargin: '2400px'` to useInfiniteScroll (was the 600px
  default). The masonry sentinel sits at the bottom of the container,
  whose height = MAX(column heights). A tall image in one column pushes
  the sentinel ~2× viewport below where the user is actually reading
  (the bottom of the SHORTER columns). 2400px ≈ 2-3 screen-heights of
  pre-emptive trigger, comfortable for typical tall manga heights.

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

frontend/src/components/discovery/MasonryGrid.vue

@@ -79,9 +79,18 @@ function aspectStyle(item) {
   return { aspectRatio: `${w} / ${h}` }
 }
 
+// Larger rootMargin than the composable default (600px) because the
+// sentinel sits at the BOTTOM of the masonry container, whose height is
+// the MAX of the column heights. A single tall image (long manga page,
+// panorama) in one column pushes the sentinel way past the visible
+// bottom of the SHORTER columns — the user reads the short-column
+// bottoms long before the sentinel comes into view, and load-more
+// fires too late. 2400px ≈ 2-3 screen-heights of pre-emptive trigger,
+// comfortably covering typical tall-image heights. Operator-flagged
+// 2026-05-30.
 useInfiniteScroll(sentinelEl, () => {
   if (props.hasMore && !props.loading) emit('load-more')
-})
+}, { rootMargin: '2400px' })
 </script>
 
 <style scoped>