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/SuggestionsPanel.vue

@@ -51,12 +51,19 @@ import { useModalStore } from '../../stores/modal.js'
 import SuggestionsCategoryGroup from './SuggestionsCategoryGroup.vue'
 import AliasPickerDialog from './AliasPickerDialog.vue'
 
-const props = defineProps({ imageId: { type: Number, required: true } })
+const props = defineProps({
+  imageId: { type: Number, required: true },
+  // The tagging host whose chip rail to refresh after an accept. Defaults to
+  // the modal store (image modal); the Explore workspace passes its anchor host
+  // so the same panel refreshes the right surface. See TagPanel.
+  host: { type: Object, default: null },
+})
 // 'accepted' lets the parent return focus to the tag input after a suggestion is
 // applied (operator-asked 2026-06-08).
 const emit = defineEmits(['accepted'])
 const store = useSuggestionsStore()
-const modal = useModalStore()
+const modalStore = useModalStore()
+const host = props.host || modalStore
 
 // 'artist' (FC-2d-vii-c) and 'copyright' (2026-06-01) retired as
 // suggestion categories. Only 'character' remains as a people-style
@@ -84,7 +91,7 @@ watch(() => props.imageId, (id) => {
 async function onAccept(s) {
   try {
     await store.accept(s)
-    await modal.reloadTags()
+    await host.reloadTags()
     emit('accepted')
   } catch (e) {
     toast({ text: `Accept failed: ${e.message}`, type: 'error' })
@@ -98,7 +105,7 @@ async function onAliasConfirm(canonicalTagId) {
   try {
     await store.aliasAccept(aliasTarget.value, canonicalTagId)
     aliasDialog.value = false
-    await modal.reloadTags()
+    await host.reloadTags()
     emit('accepted')
   } catch (e) {
     toast({ text: `Alias failed: ${e.message}`, type: 'error' })