chore(modal): drop the ?image=N soft-compat from G5.4

Operator confirmed they have no existing ?image=N bookmarks to
preserve, so the soft-compat read on initial mount + router.replace
strip is dead weight. The modal is now purely a Pinia overlay — no
URL involvement on open OR initial mount.

Drops 33 lines plus the now-unused vue-router imports in both
ArtistGalleryTab (entire onMounted gone) and GalleryView (just the
?image=N block; the post_id/tag_id filter handling stays).

frontend/src/components/artist/ArtistGalleryTab.vue

@@ -11,9 +11,6 @@
 </template>
 
 <script setup>
-import { onMounted } from 'vue'
-import { useRoute, useRouter } from 'vue-router'
-
 import { useArtistStore } from '../../stores/artist.js'
 import { useModalStore } from '../../stores/modal.js'
 import MasonryGrid from '../discovery/MasonryGrid.vue'
@@ -24,21 +21,6 @@ const props = defineProps({
 
 const store = useArtistStore()
 const modal = useModalStore()
-const route = useRoute()
-const router = useRouter()
-
-onMounted(() => {
-  // Audit 2026-06-02: same overlay treatment as GalleryView —
-  // honor `?image=N` once for backward-compat, then strip the
-  // query and let the modal live in Pinia-only state.
-  const initial = parseInt(route.query.image, 10)
-  if (!isNaN(initial)) {
-    modal.open(initial)
-    const q = { ...route.query }
-    delete q.image
-    router.replace({ query: q })
-  }
-})
 
 function openImage (id) {
   modal.open(id)