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/ToastContainer.tsx

@@ -2,12 +2,18 @@ import { useEffect, useState } from 'react'
 import { CheckCircle, XCircle, Info, AlertCircle, X } from 'lucide-react'
 import clsx from 'clsx'
 
+export interface ToastAction {
+  label: string
+  onClick: () => void
+}
+
 export interface Toast {
   id: string
   type: 'success' | 'error' | 'info' | 'warning'
   title: string
   message?: string
   duration?: number
+  action?: ToastAction
 }
 
 // Global toast state (in production, use Zustand or Context)
@@ -15,22 +21,34 @@ let toastListeners: ((toasts: Toast[]) => void)[] = []
 let toastList: Toast[] = []
 
 export const toast = {
-  success: (title: string, message?: string) => addToast('success', title, message),
-  error: (title: string, message?: string) => addToast('error', title, message),
-  info: (title: string, message?: string) => addToast('info', title, message),
-  warning: (title: string, message?: string) => addToast('warning', title, message),
+  success: (title: string, message?: string, action?: ToastAction) =>
+    addToast('success', title, message, 5000, action),
+  error: (title: string, message?: string, action?: ToastAction) =>
+    addToast('error', title, message, 5000, action),
+  info: (title: string, message?: string, action?: ToastAction) =>
+    addToast('info', title, message, 5000, action),
+  warning: (title: string, message?: string, action?: ToastAction) =>
+    addToast('warning', title, message, 5000, action),
 }
 
-function addToast(type: Toast['type'], title: string, message?: string, duration = 5000) {
-  const id = Date.now().toString()
-  const newToast: Toast = { id, type, title, message, duration }
+function addToast(
+  type: Toast['type'],
+  title: string,
+  message?: string,
+  duration = 5000,
+  action?: ToastAction
+) {
+  const id = Date.now().toString() + Math.random().toString(36).slice(2, 6)
+  const newToast: Toast = { id, type, title, message, duration, action }
   toastList = [...toastList, newToast]
   toastListeners.forEach(listener => listener(toastList))
-  
-  // Auto-remove after duration
+
+  // Auto-remove after duration. Toasts with an action get a longer window
+  // so the user has time to actually click Undo.
+  const removeAfter = action ? Math.max(duration, 8000) : duration
   setTimeout(() => {
     removeToast(id)
-  }, duration)
+  }, removeAfter)
 }
 
 function removeToast(id: string) {
@@ -82,6 +100,17 @@ export function ToastContainer() {
               <div className="mt-0.5 text-sm text-text-muted">{toast.message}</div>
             )}
           </div>
+          {toast.action && (
+            <button
+              onClick={() => {
+                toast.action!.onClick()
+                removeToast(toast.id)
+              }}
+              className="pointer-events-auto self-center rounded border border-border bg-surface px-2 py-1 text-xs font-medium text-text hover:bg-surface-2"
+            >
+              {toast.action.label}
+            </button>
+          )}
           <button
             onClick={() => removeToast(toast.id)}
             className="pointer-events-auto rounded p-0.5 text-text-muted hover:bg-surface-offset hover:text-text"