refactor(I4): extract useInfiniteScroll composable; migrate 6 consumers

The IntersectionObserver-on-a-sentinel infinite-scroll pattern was copy-
pasted in 7 places. New composables/useInfiniteScroll(sentinelRef, cb)
owns the observer lifecycle (attach on mount, re-attach when the ref
changes, disconnect on unmount). Migrated GalleryGrid, MasonryGrid
(gallery+showcase), ArtistPostsTab, ArtistsView, TagsView, SeriesManageView.
PostsView left manual on purpose — its anchored mode does bidirectional
scroll-position preservation that doesn't fit the simple composable.

I4(a) (timestamps) is effectively already done: the UTC displays were
converted earlier; the remaining toLocaleString uses already render local
time, so they're left as-is rather than churned for format-only consistency.

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

frontend/src/views/SeriesManageView.vue

@@ -61,15 +61,15 @@
 </template>
 
 <script setup>
-import { onMounted, onUnmounted, ref } from 'vue'
+import { onMounted, ref } from 'vue'
 import { useRoute } from 'vue-router'
 import { useSeriesManageStore, moveItem } from '../stores/seriesManage.js'
+import { useInfiniteScroll } from '../composables/useInfiniteScroll.js'
 
 const route = useRoute()
 const store = useSeriesManageStore()
 const dragFrom = ref(null)
 const sentinel = ref(null)
-let observer = null
 
 function onDrop(toIdx) {
   if (dragFrom.value === null || dragFrom.value === toIdx) return
@@ -80,15 +80,12 @@ function onDrop(toIdx) {
   store.reorder(ordered)
 }
 
+useInfiniteScroll(sentinel, () => store.loadPicker())
+
 onMounted(async () => {
   await store.load(parseInt(route.params.tagId, 10))
   await store.loadPicker(true)
-  observer = new IntersectionObserver(([e]) => {
-    if (e.isIntersecting) store.loadPicker()
-  }, { rootMargin: '600px' })
-  if (sentinel.value) observer.observe(sentinel.value)
 })
-onUnmounted(() => observer && observer.disconnect())
 </script>
 
 <style scoped>