fix: mobile modal UX improvements (delete confirm, browser-back, z-index) (#206)

- Replace browser confirm() with ConfirmModal for delete confirmation
- Add dedicated history entry for delete dialog so browser back dismisses it
- Track unsaved-changes warning source to restore correct context on cancel
- Add overlayClassName prop to ConfirmModal for nested z-index layering
- Add .nested-confirm CSS class for proper modal stacking
- Add i18n keys for delete confirmation dialog (EN + DE)

Closes #202

frontend/src/styles.css

@@ -5497,6 +5497,10 @@ a.about-version-link:hover {
 	background: rgba(0, 0, 0, 0.6);
 }
 
+.modal-overlay.nested-confirm {
+	z-index: 1200;
+}
+
 /* =============================================================================
    Shared Schedule Page (Public)
    ============================================================================= */