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/router.js

@@ -19,20 +19,21 @@ const routes = [
   { path: '/', redirect: FRONT_DOOR },
 
   // 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' } },
+  { path: '/showcase', name: 'showcase', component: ShowcaseView, meta: { title: 'Showcase', navOrder: 10 } },
+  { path: '/gallery',  name: 'gallery',  component: GalleryView,  meta: { title: 'Gallery', navOrder: 20  } },
+  // Explore: a 3-pane tagging workspace — walk an image's visual neighbours
+  // (left) while tagging the focused image (center viewer + modal-parity tag
+  // rail). Optional anchor param — the bare /explore nav entry SEEDS a random
+  // image so the tab kick-starts a rabbit hole. navOrder pins it after Gallery.
+  { path: '/explore/:imageId?', name: 'explore', component: ExploreView, meta: { title: 'Explore', navOrder: 25 } },
   // 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).
-  { path: '/browse', name: 'browse', component: BrowseView, meta: { title: 'Browse' } },
+  { path: '/browse', name: 'browse', component: BrowseView, meta: { title: 'Browse', navOrder: 30 } },
   // Artist detail — no meta.title (reached by clicking an artist, not nav).
   { path: '/artist/:slug', name: 'artist', component: ArtistView },
   // Series browse — a nav entry (meta.title).
-  { path: '/series', name: 'series', component: SeriesView, meta: { title: 'Series' } },
+  { path: '/series', name: 'series', component: SeriesView, meta: { title: 'Series', navOrder: 40 } },
   // Series management — no meta.title (reached from a series card/tag).
   { path: '/series/:tagId', name: 'series-manage', component: SeriesManageView },
   // Series reader — immersive (no top nav, no meta.title).
@@ -40,7 +41,7 @@ const routes = [
 
   // FC-3: subscription backbone — purely management (sources/downloads),
   // distinct from the Browse hub.
-  { path: '/subscriptions', name: 'subscriptions', component: SubscriptionsView, meta: { title: 'Subscriptions' } },
+  { path: '/subscriptions', name: 'subscriptions', component: SubscriptionsView, meta: { title: 'Subscriptions', navOrder: 50 } },
 
   // Settings — config, pinned to the right of the nav (TopNav special-cases it).
   { path: '/settings', name: 'settings', component: SettingsView, meta: { title: 'Settings' } },