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/de.json

@@ -26,7 +26,9 @@
     "nameRequired": "Name ist erforderlich",
     "contentRequired": "Inhalt ist erforderlich",
     "all": "Alle",
-    "unknownError": "Unbekannter Fehler"
+    "unknownError": "Unbekannter Fehler",
+    "confirm": "Bestätigen",
+    "undo": "Rückgängig"
   },
 
   "nav": {
@@ -155,6 +157,7 @@
     "renameListPrompt": "Neuer Listen-Name:",
     "deleteListConfirm": "Liste \"{{name}}\" und alle Artikel löschen?",
     "deletedListToast": "Liste gelöscht.",
+    "itemDeletedToast": "\"{{name}}\" entfernt.",
     "itemsRemovedToast": "{{count}} Artikel entfernt.",
     "clearChecked": "Abgehakt löschen ({{count}})",
     "itemNamePlaceholder": "Artikel hinzufügen…",