feat: close modals with browser back button on mobile (#257)

* feat: close modals with browser back button on mobile

Create reusable useModalHistory hook that pushes history state when a
modal opens and listens for popstate to close it. Apply to ReportModal,
ClearMissedConfirm, ExportModal, ImportConfirm, and all modals using
ConfirmModal/ShareDialog/Auth/ExportModal base components. Escape key
handling was already in place for desktop.

Closes #253

* fix: update tests for renamed button labels and missing useModalHistory mock

frontend/src/pages/DashboardPage.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 import { ConfirmModal, MedicationAvatar } from "../components";
 import { useAuth } from "../components/Auth";
 import { useAppContext } from "../context";
+import { useModalHistory } from "../hooks";
 import { formatNumber, getExpiryClass, getSystemLocale } from "../utils/formatters";
 import { expandDoseIds, getStockStatus, isDoseDismissed } from "../utils/schedule";
 import {
@@ -75,6 +76,8 @@ export function DashboardPage() {
 		loadSettings,
 	} = useAppContext();
 
+	useModalHistory(showClearMissedConfirm, "clearMissed", () => setShowClearMissedConfirm(false));
+
 	// Get structured reminder data
 	const reminderData = getReminderStatusData(
 		settings.reminderDaysBefore,