feat(explore): Explore view + tag-gap closing + modal meta/download (#94b–d, #4a/b)

Cluster C frontend, milestone #94.

#94b Explore walk: new /explore/:imageId route + ExploreView + explore store.
Anchor (reuse /api/gallery/image), neighbour grid (reuse /api/gallery/similar,
24), click a neighbour to re-anchor; in-memory breadcrumb that trims on
backtrack (route is the source of truth). Empty/loading/error + no-embedding
states.

#94c tag-gap closing: components/explore/TagGapPanel — fetches
/api/images/cluster/tag-gaps for the anchor+neighbours, a consensus-threshold
slider (default 60%), per gap shows present/total + the missing thumbnails +
'Apply to N missing' → /api/tags/images/bulk/tags (source manual) → re-fetch.

#94d entry points: 'Explore' button in the modal RelatedStrip; the TopNav entry
comes free from the route's meta.title.

#4a metadata HUD + #4b split Download: new modal ImageMetaBar (always-on, above
ProvenancePanel) shows dimensions/size/type and a split Download button
(default Download, chevron → Copy link via utils/clipboard — no clipboard-image,
rule 95).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

frontend/src/stores/explore.js

@@ -0,0 +1,92 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import { useApi } from '../composables/useApi.js'
+import { useInflightToken } from '../composables/useInflightToken.js'
+
+// Explore (#94): anchor on an image, walk its visual neighbours (pgvector
+// SigLIP via /api/gallery/similar), keep an in-memory breadcrumb of the walked
+// path, and surface cluster-consensus tag gaps for the current set. The ROUTE
+// (`/explore/:imageId`) is the source of truth for the anchor; the view calls
+// anchorOn() on every param change and the store reconciles the trail.
+const NEIGHBOR_LIMIT = 24
+
+export const useExploreStore = defineStore('explore', () => {
+  const api = useApi()
+
+  const anchor = ref(null)       // /api/gallery/image/<id> payload
+  const neighbors = ref([])      // [{id, thumbnail_url, ...}]
+  const breadcrumb = ref([])     // [{id, thumbnail_url}] walked path
+  const loading = ref(false)
+  const error = ref(null)
+
+  const inflight = useInflightToken()
+
+  async function anchorOn (id) {
+    const numId = Number(id)
+    if (!Number.isInteger(numId) || numId <= 0) return
+    inflight.cancel()
+    const t = inflight.claim()
+    loading.value = true
+    error.value = null
+    try {
+      const detail = await api.get(`/api/gallery/image/${numId}`)
+      if (!t.isCurrent()) return
+      anchor.value = detail
+      _reconcileTrail(numId, detail.thumbnail_url)
+      // Videos / not-yet-embedded images have no neighbours — leave the grid
+      // empty and let the view explain why (anchor.has_embedding === false).
+      if (detail.has_embedding) {
+        const body = await api.get('/api/gallery/similar', {
+          params: { similar_to: numId, limit: NEIGHBOR_LIMIT },
+        })
+        if (!t.isCurrent()) return
+        neighbors.value = body.images || []
+      } else {
+        neighbors.value = []
+      }
+    } catch (e) {
+      if (t.isCurrent()) { error.value = e.message; neighbors.value = [] }
+    } finally {
+      if (t.isCurrent()) loading.value = false
+    }
+  }
+
+  // Forward walk appends; navigating to an id already in the trail (a
+  // breadcrumb click, or a loop back) TRIMS to it — so the route stays the
+  // single source of truth and the crumb bar never grows stale branches.
+  function _reconcileTrail (id, thumbnailUrl) {
+    const idx = breadcrumb.value.findIndex((c) => c.id === id)
+    if (idx >= 0) breadcrumb.value = breadcrumb.value.slice(0, idx + 1)
+    else breadcrumb.value = [...breadcrumb.value, { id, thumbnail_url: thumbnailUrl }]
+  }
+
+  function reset () {
+    inflight.cancel()
+    anchor.value = null
+    neighbors.value = []
+    breadcrumb.value = []
+    error.value = null
+    loading.value = false
+  }
+
+  // The consensus set = the anchor plus its neighbours.
+  const clusterIds = computed(() => {
+    const ids = anchor.value ? [anchor.value.id] : []
+    return ids.concat(neighbors.value.map((n) => n.id))
+  })
+
+  // Quick id→thumbnail lookup so the gap panel can render the "missing"
+  // images without another fetch (they're already in the cluster).
+  const thumbById = computed(() => {
+    const map = {}
+    if (anchor.value) map[anchor.value.id] = anchor.value.thumbnail_url
+    for (const n of neighbors.value) map[n.id] = n.thumbnail_url
+    return map
+  })
+
+  return {
+    anchor, neighbors, breadcrumb, loading, error,
+    clusterIds, thumbById, NEIGHBOR_LIMIT,
+    anchorOn, reset,
+  }
+})