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/hooks/useKeyboardShortcuts.ts

@@ -4,6 +4,7 @@ import { usePhotoStore } from '../store/photoStore'
 import { photos as photosApi, heaps as heapsApi, type Heap } from '../services/api'
 import { HEAPS_QUERY_KEY } from './useHeapsQuery'
 import { toast } from '../components/ToastContainer'
+import { registerUndoable, useUndoStore } from '../store/undoStore'
 
 interface KeyboardShortcutsProps {
   onToggleLeftSidebar: () => void
@@ -106,7 +107,29 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
     if (ids.length === 0) return
 
     if (ids.length === 1) {
-      updateMutation.mutate({ id: ids[0], data })
+      const id = ids[0]
+      updateMutation.mutate(
+        { id, data },
+        {
+          onSuccess: () => {
+            // Only the discard/restore subset of single-photo updates is
+            // undoable today — rating and color round-trip cleanly enough
+            // that the manual fix is faster than maintaining per-photo
+            // previous-value snapshots.
+            if (data.is_discarded === true) {
+              registerUndoable('Discarded 1 photo', async () => {
+                await photosApi.bulkRestore([id])
+                invalidatePhotoQueries()
+              })
+            } else if (data.is_discarded === false) {
+              registerUndoable('Restored 1 photo', async () => {
+                await photosApi.bulkDiscard([id])
+                invalidatePhotoQueries()
+              })
+            }
+          },
+        }
+      )
       return
     }
 
@@ -118,9 +141,29 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
       bulkColorMutation.mutate({ ids, color: data.color_label })
     }
     if (data.is_discarded === true) {
-      bulkDiscardMutation.mutate(ids)
+      bulkDiscardMutation.mutate(ids, {
+        onSuccess: () => {
+          registerUndoable(
+            `Discarded ${ids.length} photo${ids.length === 1 ? '' : 's'}`,
+            async () => {
+              await photosApi.bulkRestore(ids)
+              invalidatePhotoQueries()
+            }
+          )
+        },
+      })
     } else if (data.is_discarded === false) {
-      bulkRestoreMutation.mutate(ids)
+      bulkRestoreMutation.mutate(ids, {
+        onSuccess: () => {
+          registerUndoable(
+            `Restored ${ids.length} photo${ids.length === 1 ? '' : 's'}`,
+            async () => {
+              await photosApi.bulkDiscard(ids)
+              invalidatePhotoQueries()
+            }
+          )
+        },
+      })
     }
   }
 
@@ -223,6 +266,26 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
   useHotkeys('tab', onToggleLeftSidebar, HK_OPTS)
   useHotkeys('i', onToggleRightSidebar, { ...HK_OPTS, enabled: !isPreview })
 
+  // Cmd/Ctrl+Z → pop the most recent undoable action and reverse it.
+  // Bound at the global level so it works in both grid and preview modes.
+  useHotkeys(
+    'mod+z',
+    async () => {
+      const entry = useUndoStore.getState().pop()
+      if (!entry) {
+        toast.info('Nothing to undo')
+        return
+      }
+      try {
+        await entry.undo()
+      } catch (e: any) {
+        useUndoStore.getState().push({ label: entry.label, undo: entry.undo })
+        toast.error('Undo failed', e?.message || 'Unknown error')
+      }
+    },
+    HK_OPTS
+  )
+
   // Search focus (/ or Cmd/Ctrl+F).
   const focusSearch = () => {
     const el = document.getElementById('topbar-search') as HTMLInputElement | null