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/components/artist/ArtistPostsTab.vue

@@ -28,10 +28,11 @@
 </template>
 
 <script setup>
-import { onMounted, onUnmounted, ref, watch } from 'vue'
+import { onMounted, ref, watch } from 'vue'
 import { RouterLink } from 'vue-router'
 
 import { usePostsStore } from '../../stores/posts.js'
+import { useInfiniteScroll } from '../../composables/useInfiniteScroll.js'
 import PostCard from '../posts/PostCard.vue'
 
 const props = defineProps({
@@ -42,7 +43,6 @@ defineEmits(['switch-tab'])
 
 const store = usePostsStore()
 const sentinel = ref(null)
-let observer = null
 
 async function reload () {
   await store.loadInitial({ artist_id: props.artistId, platform: null })
@@ -50,19 +50,9 @@ async function reload () {
 
 watch(() => props.artistId, reload)
 
-onMounted(async () => {
-  await reload()
-  observer = new IntersectionObserver((entries) => {
-    if (entries.some(e => e.isIntersecting)) {
-      store.loadMore()
-    }
-  }, { rootMargin: '400px 0px' })
-  if (sentinel.value) observer.observe(sentinel.value)
-})
+useInfiniteScroll(sentinel, () => store.loadMore(), { rootMargin: '400px 0px' })
 
-onUnmounted(() => {
-  if (observer) observer.disconnect()
-})
+onMounted(reload)
 </script>
 
 <style scoped>