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/ConfirmModal.tsx

@@ -47,7 +47,7 @@ export function ConfirmModal({
 			}}
 		>
 			<div
-				className="modal-content"
+				className="modal-content confirm-modal"
 				onClick={(e) => e.stopPropagation()}
 				onKeyDown={(e) => e.stopPropagation()}
 				style={{ maxWidth: "450px" }}