fix(audit-g5): centroid version DB-as-truth + modal as overlay

Closes the last two findings from the 2026-06-02 audit (G5.1 + G5.4).

G5.1 — Centroid version no longer drifts:

CentroidService now reads MLSettings.embedder_model_version (the DB
row tag_and_embed already writes from) for both the centroid model-
version stamp and the drift-detection comparison. Previously the
centroid sites imported MODEL_VERSION from env, so the version stamped
on centroids could disagree with the version stamped on the embeddings
they were built from. By construction those now match, so list_drifted
won't silently miss the env-vs-DB drift case.

embedder.py keeps MODEL_VERSION as an env-driven constant for the
actual model loader — that's a different concern (which weights are
loaded) from the version-stamp that gets persisted alongside data.

G5.4 — Modal is a Pinia-only overlay:

The previous URL↔modal sync in GalleryView and ArtistGalleryTab
leaked the modal across route changes (RouterLink to /artist/<slug>
left the modal mounted on top of the new route) and re-opened it
on history back/forward with stale ?image=N entries.

Now: openImage() just calls modal.open(id) — no URL push.
GalleryView's dead closeImage helper is deleted. A route.name
watcher in App.vue closes the modal whenever the route changes,
which auto-fixes RouterLink-in-modal and back/forward.

Backward-compat: ?image=N is still honored on initial mount as a
one-shot deep-link opener, then router.replace strips the query so
the URL doesn't re-trigger and no extra history entry is added.
Existing bookmarks / shared URLs keep working; new opens stay
Pinia-only.

frontend/src/App.vue

@@ -9,7 +9,9 @@
 </template>
 
 <script setup>
-import { onMounted, ref } from 'vue'
+import { onMounted, ref, watch } from 'vue'
+import { useRoute } from 'vue-router'
+
 import AppShell from './components/AppShell.vue'
 import AppSnackbar from './components/AppSnackbar.vue'
 import ImageViewer from './components/modal/ImageViewer.vue'
@@ -17,9 +19,20 @@ import { useModalStore } from './stores/modal.js'
 
 const modal = useModalStore()
 const snackbar = ref(null)
+const route = useRoute()
 
 onMounted(() => {
   // Expose snackbar via a simple global so stores can call it without props.
   window.__fcToast = (opts) => snackbar.value?.open(opts)
 })
+
+// Audit 2026-06-02: the modal is an overlay, not a page. When the
+// route changes (RouterLink inside the modal, history back/forward,
+// programmatic push from any view), close the modal so it doesn't
+// hover over a different route. Watching route.name (not the path)
+// keeps within-route nav like /artist/foo → /artist/bar from
+// dismissing the modal mid-browse.
+watch(() => route.name, () => {
+  if (modal.isOpen) modal.close()
+})
 </script>