feat(showcase): IR-parity R-key shuffle + stagger entry animation; fix(cleanup): min-dim Delete swallowed crypto.subtle TypeError on plain HTTP

**showcase R-key + entry animation**

Restores two behaviors lost during the FC-2 IR→Vue port. Operator-flagged
2026-05-27.

- ShowcaseView listens for keydown 'r'/'R' on window. Triggers
  `store.shuffle()`. Skips when an input/textarea/contenteditable is
  focused or a Vuetify overlay is open (the dialog/menu sets
  `.v-overlay--active` on the body).
- MasonryGrid gains an opt-in `animateFromIndex` prop (default
  `Number.POSITIVE_INFINITY` = off). When set, items with index ≥ the
  threshold animate in with a stagger fade-in: 12px translateY,
  0.25s ease, 60ms per item, capped by `prefers-reduced-motion`.
  Stagger uses original-items-array index (resolved via an `idxById`
  Map) so the reading order is preserved even after the masonry
  distributes items across columns.
- ShowcaseView watches `store.images.length`: shrink-or-zero baseline
  ⇒ `animateFromIndex=0` (animate everything on initial load /
  shuffle); grow ⇒ baseline=prevCount (animate only the appended
  tail on infinite-scroll). Other MasonryGrid consumers (ArtistView's
  Gallery tab) don't pass the prop, so they keep their current
  no-animation behavior.

Direct port of IR's `app/static/js/showcase.js` keyboard handler +
`app/static/style.css` itemFadeIn keyframe.

**min-dim Delete: crypto.subtle TypeError fix**

The Delete button on the Cleanup → Minimum Dimensions card was
silently no-op'ing. Root cause: `crypto.subtle` is Secure-Context-gated
(undefined on plain-HTTP origins per the homelab posture). The card's
`onDeleteClick` computed the Tier-C confirm token via
`crypto.subtle.digest('SHA-256', ...)`, which threw TypeError before
`showModal.value = true`. The promise rejected, the click handler had
no `.catch`, the modal never opened — exactly the operator's reported
symptom.

Same shape as the v26.05.26.0 `navigator.clipboard` fix on the
ErrorDetailModal Copy button.

Fix: backend `/api/cleanup/min-dimension/preview` now returns
`confirm_token` (the canonical `delete-min-dim-<sha8>` string) in its
response. Frontend reads it from the preview response and feeds the
8-char suffix to DestructiveConfirmModal's `runId` prop — no
client-side crypto needed. Single source of truth.

Integration test `test_min_dimension_preview_returns_count` pinned to
also assert `body["confirm_token"]` matches the server-side compute.

frontend/src/views/ShowcaseView.vue

@@ -18,6 +18,7 @@
       :items="store.images"
       :loading="store.loading"
       :has-more="store.hasMore"
+      :animate-from-index="animateFromIndex"
       @load-more="store.fetchPage()"
       @open="openImage"
     />
@@ -25,17 +26,52 @@
 </template>
 
 <script setup>
-import { onMounted } from 'vue'
-import { useShowcaseStore } from '../stores/showcase.js'
-import { useModalStore } from '../stores/modal.js'
+import { onMounted, onUnmounted, ref, watch } from 'vue'
+
 import MasonryGrid from '../components/discovery/MasonryGrid.vue'
+import { useModalStore } from '../stores/modal.js'
+import { useShowcaseStore } from '../stores/showcase.js'
 
 const store = useShowcaseStore()
 const modal = useModalStore()
 
-onMounted(() => { if (store.images.length === 0) store.fetchPage() })
+// Track when items were appended vs replaced so MasonryGrid only animates
+// items new to the current batch (mirrors IR's behavior: animate on
+// initial load and on shuffle, but skip silent infinite-scroll appends).
+const animateFromIndex = ref(0)
+let prevCount = 0
+watch(() => store.images.length, (newCount) => {
+  if (newCount < prevCount || prevCount === 0) {
+    // Reset (shuffle) or initial load — animate everything from 0.
+    animateFromIndex.value = 0
+  } else {
+    // Append — only animate the newly-added tail.
+    animateFromIndex.value = prevCount
+  }
+  prevCount = newCount
+})
 
 function openImage(id) {
   modal.open(id)
 }
+
+// IR-parity keyboard shuffle: press R anywhere on the page (not inside a
+// text input, not while a Vuetify overlay is open) to reshuffle.
+function onKeydown(e) {
+  const t = e.target
+  if (t && (t.tagName === 'INPUT' || t.tagName === 'TEXTAREA' || t.isContentEditable)) return
+  // Vuetify marks the active overlay (v-dialog, v-menu) on the body when
+  // open. Skip shuffle when a modal is in the way.
+  if (document.querySelector('.v-overlay--active')) return
+  if (e.key === 'r' || e.key === 'R') {
+    e.preventDefault()
+    store.shuffle()
+  }
+}
+
+onMounted(() => {
+  if (store.images.length === 0) store.fetchPage()
+  window.addEventListener('keydown', onKeydown)
+})
+onUnmounted(() => window.removeEventListener('keydown', onKeydown))
 </script>