fix(ux): replace native confirm() dialogs, add undo-toast, fix prefers-reduced-motion

- Replace all 13 native confirm() calls with confirmModal() across 7 page modules
- Add confirmModal() to modal.js (Promise-based, danger variant, focus management)
- Fix double-confirm bug in contacts.js and budget.js (modal + deleteContact/deleteEntry)
- Extend showToast() with onUndo callback and max-3-toast limit
- Implement optimistic undo-toast (4s window) for shopping item and bulk-checked delete
- Add prefers-reduced-motion guard to btnSuccess() and btnError() in modal.js
- Add btn--error-static CSS class as motion-reduced fallback for btnError()
- Add toast__undo button styles to layout.css
- Add common.confirm and common.undo i18n keys (de, en, it, sv)
- Add shopping.itemDeletedToast i18n key (de, en, it, sv)

public/locales/en.json

@@ -26,7 +26,9 @@
     "nameRequired": "Name is required",
     "contentRequired": "Content is required",
     "all": "All",
-    "unknownError": "Unknown error"
+    "unknownError": "Unknown error",
+    "confirm": "Confirm",
+    "undo": "Undo"
   },
 
   "nav": {
@@ -155,6 +157,7 @@
     "renameListPrompt": "New list name:",
     "deleteListConfirm": "Delete list \"{{name}}\" and all items?",
     "deletedListToast": "List deleted.",
+    "itemDeletedToast": "\"{{name}}\" removed.",
     "itemsRemovedToast": "{{count}} items removed.",
     "clearChecked": "Remove checked ({{count}})",
     "itemNamePlaceholder": "Add item…",