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/composables/useInfiniteScroll.js

@@ -0,0 +1,28 @@
+import { onMounted, onUnmounted, watch } from 'vue'
+
+// Shared IntersectionObserver-on-a-sentinel infinite-scroll lifecycle.
+// Pass a ref to the sentinel element and a callback to fire when it scrolls
+// into view; the callback owns its own guard (e.g. `if (store.hasMore &&
+// !store.loading) store.loadMore()`). Re-attaches if the sentinel ref
+// changes (e.g. it's behind a v-if) and disconnects on unmount.
+export function useInfiniteScroll (sentinelRef, onIntersect, { rootMargin = '600px' } = {}) {
+  let observer = null
+
+  function teardown () {
+    if (observer) { observer.disconnect(); observer = null }
+  }
+  function attach () {
+    teardown()
+    if (!sentinelRef.value) return
+    observer = new IntersectionObserver(([entry]) => {
+      if (entry.isIntersecting) onIntersect()
+    }, { rootMargin })
+    observer.observe(sentinelRef.value)
+  }
+
+  onMounted(attach)
+  watch(sentinelRef, attach)
+  onUnmounted(teardown)
+
+  return { attach, teardown }
+}