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/components/Auth.tsx

@@ -756,7 +756,7 @@ export function UserProfile({ onClose }: { onClose?: () => void }) {
 
 				<div className="profile-actions">
 					<button type="button" className="btn btn-ghost" onClick={onClose}>
-						{t("common.cancel", "Cancel")}
+						{t("common.close", "Close")}
 					</button>
 					<button type="submit" className="btn btn-primary" disabled={loading || !hasChanges}>
 						{loading ? t("common.saving", "Saving...") : t("auth.updatePassword", "Update Password")}