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/store/undoStore.ts

@@ -0,0 +1,71 @@
+import { create } from 'zustand'
+import { toast } from '../components/ToastContainer'
+
+const MAX_STACK = 20
+
+export interface UndoEntry {
+  id: string
+  /** Short description of what happened, e.g. "Discarded 12 photos". */
+  label: string
+  /** Function that reverses the action. May be async; errors should be
+   *  surfaced via toast.error from inside the function. */
+  undo: () => void | Promise<void>
+}
+
+interface UndoStore {
+  stack: UndoEntry[]
+  /** Push a new entry. Caps the stack at MAX_STACK by dropping the oldest. */
+  push: (entry: Omit<UndoEntry, 'id'>) => void
+  /** Pop the most recent entry. Returns null when the stack is empty. */
+  pop: () => UndoEntry | null
+  clear: () => void
+}
+
+export const useUndoStore = create<UndoStore>((set, get) => ({
+  stack: [],
+
+  push: (entry) => {
+    const id = Date.now().toString() + Math.random().toString(36).slice(2, 6)
+    set((s) => {
+      const next = [...s.stack, { ...entry, id }]
+      if (next.length > MAX_STACK) next.shift()
+      return { stack: next }
+    })
+  },
+
+  pop: () => {
+    const stack = get().stack
+    if (stack.length === 0) return null
+    const last = stack[stack.length - 1]
+    set({ stack: stack.slice(0, -1) })
+    return last
+  },
+
+  clear: () => set({ stack: [] }),
+}))
+
+/**
+ * Convenience: register an undoable action AND show the user a success
+ * toast with an inline Undo button. The toast and Cmd+Z hotkey both pop
+ * from the same stack so either path works.
+ */
+export function registerUndoable(label: string, undo: () => void | Promise<void>) {
+  useUndoStore.getState().push({ label, undo })
+  toast.success(label, 'Press ⌘Z to undo', {
+    label: 'Undo',
+    onClick: async () => {
+      // Pop the entry we just pushed (or whatever is now on top, if the
+      // user fired multiple actions in quick succession — Undo always
+      // reverses the most recent thing).
+      const entry = useUndoStore.getState().pop()
+      if (!entry) return
+      try {
+        await entry.undo()
+      } catch (e) {
+        // Re-push so the user can try again, and surface the failure.
+        useUndoStore.getState().push({ label: entry.label, undo: entry.undo })
+        toast.error('Undo failed', e instanceof Error ? e.message : String(e))
+      }
+    },
+  })
+}