feat(explore): 3-pane tagging workspace — gallery | viewer | tag rail

Reworks Explore from "anchor + neighbour grid + cluster tag-gap rail" into a
persistent 3-pane workspace that unfolds the image modal so you can tag while
rabbit-holing (operator concept 2026-06-26):

- LEFT  neighbour grid (larger thumbs), click = walk; breadcrumb retained.
- CENTER light viewer — reuses ImageCanvas + ImageMetaBar(:image) for the
  focused image; "Open full viewer" still launches the overlay modal.
- RIGHT  the modal's TagPanel, hosted on the anchor for modal-parity tagging
  (chips, autocomplete, suggestions + Accept, fandom-on-chip, T/"/" focus).

Reuse without destabilising the audited modal store: TagPanel and
SuggestionsPanel gain an optional `host` prop (default = modal store, so the
image modal is unchanged); the explore store implements the same small
tag-CRUD surface (current/currentImageId + reloadTags/addExistingTag/
removeTag/createAndAdd) over the anchor. ImageMetaBar gains an optional
`image` prop for the same reason.

Drops the mass/cluster tagger (TagGapPanel deleted; clusterIds/thumbById
removed) — per-image tagging feeds the per-tag reference-embedding centroid
better than bulk ops.

Nav: keep the Explore tab but bare /explore now SEEDS a random image
(GET /api/showcase?limit=1 → /explore/:id) so the tab kick-starts a rabbit
hole; explicit meta.navOrder pins nav order (Explore after Gallery) since
router.getRoutes() doesn't preserve declaration order.

Note: the backend cluster tag-gaps route/service (#94a) is now frontend-orphaned
— left in place; flag for a separate cleanup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/modal/ImageMetaBar.vue

@@ -38,8 +38,12 @@ import { useModalStore } from '../../stores/modal.js'
 import { copyText } from '../../utils/clipboard.js'
 import { toast } from '../../utils/toast.js'
 
+// `image` lets a non-modal surface (the Explore workspace) render the same
+// meta + download block for its anchor. Defaults to the modal store's current
+// image so the image modal is unchanged.
+const props = defineProps({ image: { type: Object, default: null } })
 const modal = useModalStore()
-const img = computed(() => modal.current)
+const img = computed(() => props.image ?? modal.current)
 
 function humanSize (bytes) {
   const units = ['B', 'KB', 'MB', 'GB']