feat: undo for destructive photo actions

Add a global last-action stack with toast-based "Undo" buttons and a
Cmd/Ctrl+Z hotkey for the destructive photo operations.

Reversible:
- X (discard) → bulkRestore
- U (restore) → bulkDiscard
- Drag-onto-Discarded → bulkRestore
- Drag-onto-folder (move) → move back to per-photo source folders. The
  source folder ids are snapshotted from the photos cache before the
  move runs, then grouped so multi-source moves restore correctly.
- Restore button in the discard action bar → bulkDiscard

Toast gains an optional action button (label + onClick); toasts with an
action stay visible longer so the user has time to click. The undo
store caps at 20 entries; failed undo re-pushes the entry so the user
can try again.

Not reversible (call out, document later): rating, color label, copy,
permanent delete from trash, tag changes.

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

frontend/src/components/discard/DiscardActionBar.tsx

@@ -4,9 +4,10 @@ import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { usePhotoStore } from '../../store/photoStore'
 import { useFilterStore } from '../../store/filterStore'
 import { usePhotosQuery } from '../../hooks/usePhotosQuery'
-import { discard as discardApi } from '../../services/api'
+import { discard as discardApi, photos as photosApi } from '../../services/api'
 import { toast } from '../ToastContainer'
 import { ConfirmDialog } from '../dialogs/ConfirmDialog'
+import { registerUndoable } from '../../store/undoStore'
 
 /**
  * Top-of-timeline bar visible only when the discarded filter is active.
@@ -26,7 +27,13 @@ export function DiscardActionBar() {
   const restoreMutation = useMutation({
     mutationFn: (ids: string[]) => discardApi.restore(ids),
     onSuccess: (_, ids) => {
-      toast.success('Restored', `${ids.length} photo${ids.length > 1 ? 's' : ''} restored`)
+      registerUndoable(
+        `Restored ${ids.length} photo${ids.length === 1 ? '' : 's'}`,
+        async () => {
+          await photosApi.bulkDiscard(ids)
+          queryClient.invalidateQueries({ queryKey: ['photos'] })
+        }
+      )
       clearSelection()
       queryClient.invalidateQueries({ queryKey: ['photos'] })
     },