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/router.js

@@ -2,6 +2,7 @@ import { createRouter, createWebHistory, createMemoryHistory } from 'vue-router'
 import SettingsView from './views/SettingsView.vue'
 import GalleryView from './views/GalleryView.vue'
 import ShowcaseView from './views/ShowcaseView.vue'
+import ExploreView from './views/ExploreView.vue'
 import BrowseView from './views/BrowseView.vue'
 import ArtistView from './views/ArtistView.vue'
 import SeriesView from './views/SeriesView.vue'
@@ -20,6 +21,10 @@ const routes = [
   // FC-2: image backbone
   { path: '/showcase', name: 'showcase', component: ShowcaseView, meta: { title: 'Showcase' } },
   { path: '/gallery',  name: 'gallery',  component: GalleryView,  meta: { title: 'Gallery'  } },
+  // Explore: anchor on an image, walk its visual neighbours, close tag-coverage
+  // gaps across the cluster (#94). Optional anchor param — bare /explore (the
+  // nav entry) lands on an empty state that points you at an image.
+  { path: '/explore/:imageId?', name: 'explore', component: ExploreView, meta: { title: 'Explore' } },
   // Browse hub (operator-asked 2026-06-09): Posts / Artists / Tags as tabs —
   // the three "browse the library by an axis" surfaces. One nav entry; the old
   // standalone paths redirect into the matching tab (below).