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/TopNav.vue

@@ -72,10 +72,14 @@ import PipelineStatusChip from './PipelineStatusChip.vue'
 const system = useSystemStore()
 onMounted(() => system.refreshHealth())
 
-// Same mechanism the old sidebar used: every route with a meta.title is a
-// nav entry, in router declaration order. Auto-tracks future routes.
+// Every route with a meta.title is a nav entry. Order by meta.navOrder —
+// router.getRoutes() does NOT guarantee declaration order, so explicit numbers
+// pin the sequence (e.g. Explore after Gallery). Routes without one fall to the
+// end. Auto-tracks future routes.
 const navRoutes = computed(() =>
-  router.getRoutes().filter(r => r.meta?.title)
+  router.getRoutes()
+    .filter(r => r.meta?.title)
+    .sort((a, b) => (a.meta.navOrder ?? 999) - (b.meta.navOrder ?? 999))
 )
 // Content links for the centered desktop row — everything EXCEPT Settings,
 // which is config and gets pinned to the right edge instead.