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/components/posts/PostCard.vue

@@ -200,13 +200,13 @@ async function fullImageIds () {
 }
 
 async function openModal (imageId) {
-  modal.open(imageId, { postImageIds: await fullImageIds() })
+  modal.open(imageId, { playlistIds: await fullImageIds() })
 }
 
 async function openModalAtMore () {
   const ids = await fullImageIds()
   const first = ids[visibleCount.value] ?? ids[0]
-  if (first != null) modal.open(first, { postImageIds: ids })
+  if (first != null) modal.open(first, { playlistIds: ids })
 }
 
 // --- description "Show more" (text-only, in place, only when truncated) ----