feat(ui): modal prev/next walks the current gallery filter (#1322)

The image modal cycled GLOBAL neighbours; now the gallery hands it a snapshot of
the currently-filtered, ordered id list so prev/next moves through exactly what
you're viewing — the filtered-playlist behaviour lost in the ImageRepo→FC move.
Generalized the modal store's post-scoped cycle into a `playlistIds` playlist
reused by both GalleryView and PostCard (falls back to global neighbours when no
playlist is passed, e.g. Explore's "open full viewer").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/views/GalleryView.vue

@@ -54,7 +54,10 @@ watch(() => route.query, (q) => {
 })
 
 function openImage(id) {
-  modal.open(id)
+  // Walk the current gallery filter in the modal (#1322) — prev/next moves
+  // through exactly the filtered set the operator is viewing, not global
+  // neighbours. Snapshot of the currently-loaded, filtered, ordered ids.
+  modal.open(id, { playlistIds: store.images.map((i) => i.id) })
 }
 </script>