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/components/modal/ImageViewer.vue

@@ -73,6 +73,7 @@
         </div>
 
         <aside v-if="modal.current" class="fc-viewer__side">
+          <ImageMetaBar />
           <ProvenancePanel />
           <TagPanel />
           <!-- Non-blocking: fetches its own similar set after the modal is up;
@@ -90,6 +91,7 @@ import { useModalStore } from '../../stores/modal.js'
 import { arrowNavAllowed, isTextEntry } from '../../utils/textEntry.js'
 import ImageCanvas from './ImageCanvas.vue'
 import VideoCanvas from './VideoCanvas.vue'
+import ImageMetaBar from './ImageMetaBar.vue'
 import TagPanel from './TagPanel.vue'
 import ProvenancePanel from './ProvenancePanel.vue'
 import RelatedStrip from './RelatedStrip.vue'