feat: drag photos onto a heap row to add them

Lightroom-style direct manipulation: pick up a photo (or a multi-
selection) and drop it on a heap to add it. Complements the P
shortcut without replacing it.

PhotoThumbnail
- Becomes draggable. onDragStart reads the current selection from
  the photo store at fire time: if the dragged photo is part of the
  selection, the payload is the whole selection; otherwise it's
  just that one photo. Matches Finder semantics.
- Payload uses a custom MIME (application/x-mulita-photos) so the
  drop target can recognise our drags vs. arbitrary file drags from
  the OS. Also sets text/plain so dropping outside the app shows a
  sensible "N photos" string.

HeapsPanel
- Each heap row is now a drop target. onDragOver previews the drop
  effect and highlights the row with a primary ring and a faint
  background tint. onDragLeave only clears the highlight if the
  cursor actually left the row (not just moved over a child).
- New dropMutation handles the drop: optimistic membership cache
  update so the basket affordance flips immediately, rollback on
  error from a captured `previous`, success toast naming the heap
  and the count of newly-added photos, onSettled invalidation of
  heaps + heap-photo-ids + photos so server truth re-syncs.

PhotoThumbnail's title attribute now mentions the drag affordance
alongside click/double-click/shift+click/ctrl+click hints.

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

frontend/src/components/heaps/HeapsPanel.tsx

@@ -13,6 +13,7 @@ import { useHeapsQuery, HEAPS_QUERY_KEY } from '../../hooks/useHeapsQuery'
 import { heaps as heapsApi } from '../../services/api'
 import { useFilterStore } from '../../store/filterStore'
 import { toast } from '../ToastContainer'
+import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
 
 /**
  * Heaps panel for the left sidebar. Renders the list of heaps with the
@@ -33,6 +34,9 @@ export function HeapsPanel() {
   const [expanded, setExpanded] = useState(true)
   const [creating, setCreating] = useState(false)
   const [newName, setNewName] = useState('')
+  // Which heap row is currently being hovered with a drag — used to render
+  // the drop highlight ring. Only one heap can be the target at a time.
+  const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
   const invalidate = () => {
     queryClient.invalidateQueries({ queryKey: HEAPS_QUERY_KEY })
@@ -71,6 +75,47 @@ export function HeapsPanel() {
       toast.error('Failed to delete heap', e.message || 'Unknown error'),
   })
 
+  // Drop handler: add the dragged photos to the target heap. Optimistically
+  // updates the membership cache so the basket affordance flips immediately,
+  // mirroring the keyboard P-toggle pattern.
+  const dropMutation = useMutation({
+    mutationFn: ({ heapId, photoIds }: { heapId: string; photoIds: string[] }) =>
+      heapsApi.addPhotos(heapId, photoIds),
+    onMutate: ({ heapId, photoIds }) => {
+      const key = ['heap-photo-ids', heapId] as const
+      const previous = queryClient.getQueryData<string[]>(key)
+      const set = new Set(previous ?? [])
+      photoIds.forEach((id) => set.add(id))
+      queryClient.setQueryData<string[]>(key, Array.from(set))
+      return { previous }
+    },
+    onError: (e: any, vars, ctx) => {
+      if (ctx?.previous) {
+        queryClient.setQueryData(['heap-photo-ids', vars.heapId], ctx.previous)
+      }
+      toast.error('Failed to add to heap', e.message || 'Unknown error')
+    },
+    onSuccess: (data, vars) => {
+      const heap = heaps.find((h) => h.id === vars.heapId)
+      const heapName = heap?.name ?? 'heap'
+      const added = data?.added ?? 0
+      const already = data?.already_present ?? 0
+      if (added > 0) {
+        toast.success(
+          `Added to ${heapName}`,
+          `${added} photo${added > 1 ? 's' : ''}${already > 0 ? ` (${already} already present)` : ''}`
+        )
+      } else if (already > 0) {
+        toast.info(`Already in ${heapName}`, `${already} photo${already > 1 ? 's' : ''}`)
+      }
+    },
+    onSettled: (_d, _e, vars) => {
+      queryClient.invalidateQueries({ queryKey: HEAPS_QUERY_KEY })
+      queryClient.invalidateQueries({ queryKey: ['heap-photo-ids', vars.heapId] })
+      queryClient.invalidateQueries({ queryKey: ['photos'] })
+    },
+  })
+
   const handleCreate = () => {
     const name = newName.trim()
     if (!name) return
@@ -151,15 +196,45 @@ export function HeapsPanel() {
           {heaps.map((heap) => {
             const isFiltered = filterHeapId === heap.id
             const isActive = heap.is_active
+            const isDropTarget = dropTargetId === heap.id
             return (
               <div
                 key={heap.id}
                 className={clsx(
                   'group flex cursor-pointer items-center gap-1 rounded px-2 py-1 text-[13px]',
-                  isFiltered ? 'bg-primary/20 text-primary' : 'text-text hover:bg-surface-2'
+                  isFiltered ? 'bg-primary/20 text-primary' : 'text-text hover:bg-surface-2',
+                  isDropTarget && 'ring-2 ring-primary bg-primary/10'
                 )}
                 style={{ paddingLeft: '32px' }}
                 onClick={() => setFilterHeapId(heap.id)}
+                onDragOver={(e) => {
+                  if (e.dataTransfer.types.includes(PHOTO_DRAG_MIME)) {
+                    e.preventDefault()
+                    e.dataTransfer.dropEffect = 'copy'
+                    if (dropTargetId !== heap.id) setDropTargetId(heap.id)
+                  }
+                }}
+                onDragLeave={(e) => {
+                  // Only clear if we're actually leaving this row, not just
+                  // moving over a child element.
+                  if (!e.currentTarget.contains(e.relatedTarget as Node)) {
+                    if (dropTargetId === heap.id) setDropTargetId(null)
+                  }
+                }}
+                onDrop={(e) => {
+                  e.preventDefault()
+                  setDropTargetId(null)
+                  const raw = e.dataTransfer.getData(PHOTO_DRAG_MIME)
+                  if (!raw) return
+                  try {
+                    const ids = JSON.parse(raw) as string[]
+                    if (Array.isArray(ids) && ids.length > 0) {
+                      dropMutation.mutate({ heapId: heap.id, photoIds: ids })
+                    }
+                  } catch {
+                    // Bad payload — ignore.
+                  }
+                }}
               >
                 <ShoppingBasket
                   className={clsx(