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/stores/gallery.js

@@ -2,6 +2,13 @@ import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
 
+// Operator-confirmed 2026-05-30: fetch PAGE-sized chunks instead of one
+// 50-item request so items render as each batch lands. Total initial
+// count is unchanged (PAGE * INITIAL_BATCHES = 50). Infinite-scroll also
+// pulls PAGE per trigger to keep appends progressive.
+const PAGE = 5
+const INITIAL_BATCHES = 10
+
 export const useGalleryStore = defineStore('gallery', () => {
   const api = useApi()
 
@@ -21,7 +28,13 @@ export const useGalleryStore = defineStore('gallery', () => {
     images.value = []
     dateGroups.value = []
     nextCursor.value = null
-    await loadMore()
+    // Sequentially fetch INITIAL_BATCHES chunks so items render as each
+    // batch lands rather than blocking on one big response. Stop early
+    // when the backend reports no more pages.
+    for (let i = 0; i < INITIAL_BATCHES; i++) {
+      if (i > 0 && nextCursor.value === null) break
+      await loadMore()
+    }
   }
 
   async function loadMore() {
@@ -30,7 +43,7 @@ export const useGalleryStore = defineStore('gallery', () => {
     error.value = null
     const myId = ++inflightId
     try {
-      const params = { limit: 50, ...activeFilterParam() }
+      const params = { limit: PAGE, ...activeFilterParam() }
       if (nextCursor.value) params.cursor = nextCursor.value
       const body = await api.get('/api/gallery/scroll', { params })
       if (myId !== inflightId) return  // stale response

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))