feat(ui): batch initial showcase/gallery loads into smaller chunks

Showcase and gallery were fetching the full initial batch (60 / 50
items) in a single API call. Operator-flagged 2026-05-30: items should
stream in as small batches so they render progressively rather than
blocking on one big response.

- showcase store: PAGE 60 → 5, INITIAL_BATCHES = 12 (5 × 12 = 60, same
  total). loadInitial() fetches PAGE chunks in sequence; shuffle()
  delegates. fetchPage() still returns one PAGE-sized chunk so
  infinite-scroll also pulls 5 per trigger.
- gallery store: loadMore() limit 50 → PAGE (5), INITIAL_BATCHES = 10.
  loadInitial() loops loadMore() up to INITIAL_BATCHES times, stopping
  early when nextCursor becomes null (end of data).
- ShowcaseView: onMounted calls loadInitial() instead of fetchPage().
  Gallery/setTagFilter/setPostFilter already routed through loadInitial.

Net visual effect: each ~5-item batch lands and renders independently;
combined with MasonryGrid's animateFromIndex logic, the showcase now
cascades batches in as they arrive instead of one big pop-in.

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

frontend/src/views/ShowcaseView.vue

@@ -70,7 +70,7 @@ function onKeydown(e) {
 }
 
 onMounted(() => {
-  if (store.images.length === 0) store.fetchPage()
+  if (store.images.length === 0) store.loadInitial()
   window.addEventListener('keydown', onKeydown)
 })
 onUnmounted(() => window.removeEventListener('keydown', onKeydown))