fix: optimistic membership cache so P toggle is instant and reliable

P already toggled membership in the active heap (remove if every
selected photo is a member, otherwise add the missing ones), but the
mutation only invalidate-then-refetched the heap-photo-ids cache on
success. Pressing P twice in quick succession could read the stale
cache and mis-toggle.

Both heap-toggle mutations (useKeyboardShortcuts P shortcut and the
RightSidebar Pick button) now do an optimistic update in onMutate:
- Read the current ['heap-photo-ids', heapId] cache
- Add or remove the affected ids in a Set
- Write the new array back via setQueryData
- Roll back from the captured `previous` on error
- Re-sync via invalidateQueries in onSettled (heap counts in particular
  still need server truth)

Result: the basket affordance flips the moment you press P, and a
quick second press always reads the new state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/RightSidebar.tsx

@@ -146,14 +146,29 @@ export function RightSidebar() {
         ? heapsApi.removePhotos(activeHeap.id, [activePhotoId])
         : heapsApi.addPhotos(activeHeap.id, [activePhotoId])
     },
-    onSuccess: () => {
+    // Optimistic flip so the badge / button label update instantly.
+    onMutate: ({ remove }) => {
+      if (!activeHeap || !activePhotoId) return { previous: undefined }
+      const key = ['heap-photo-ids', activeHeap.id] as const
+      const previous = queryClient.getQueryData<string[]>(key)
+      const set = new Set(previous ?? [])
+      if (remove) set.delete(activePhotoId)
+      else set.add(activePhotoId)
+      queryClient.setQueryData<string[]>(key, Array.from(set))
+      return { previous }
+    },
+    onError: (_e, _vars, ctx) => {
+      if (activeHeap && ctx?.previous) {
+        queryClient.setQueryData(['heap-photo-ids', activeHeap.id], ctx.previous)
+      }
+    },
+    onSettled: () => {
       queryClient.invalidateQueries({ queryKey: HEAPS_QUERY_KEY })
       if (activeHeap) {
         queryClient.invalidateQueries({
           queryKey: ['heap-photo-ids', activeHeap.id],
         })
       }
-      queryClient.invalidateQueries({ queryKey: ['photos'] })
     },
   })