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/styles/layout.css

@@ -1366,6 +1366,25 @@
   height: 16px;
 }
 
+.toast__undo {
+  margin-left: auto;
+  flex-shrink: 0;
+  padding: var(--space-1) var(--space-2);
+  background: transparent;
+  border: 1px solid currentColor;
+  border-radius: var(--radius-sm);
+  color: inherit;
+  font-size: var(--text-xs);
+  font-weight: var(--font-weight-semibold);
+  cursor: pointer;
+  white-space: nowrap;
+  opacity: 0.85;
+}
+
+.toast__undo:hover {
+  opacity: 1;
+}
+
 .toast--success { background-color: var(--color-success); color: var(--color-text-on-accent); }
 .toast--danger  { background-color: var(--color-danger);  color: var(--color-text-on-accent); }
 .toast--warning { background-color: var(--color-warning); color: var(--color-text-on-accent); }
@@ -1491,6 +1510,13 @@
   animation: btn-shake 0.3s ease;
 }
 
+/* prefers-reduced-motion: kein Schütteln, nur Farb-Feedback */
+.btn--error-static {
+  background-color: var(--color-danger) !important;
+  color: var(--color-text-on-accent) !important;
+  pointer-events: none;
+}
+
 .btn--success {
   background-color: var(--color-success) !important;
   color: var(--color-text-on-accent) !important;