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/ShareDialog.tsx

@@ -145,7 +145,7 @@ export function ShareDialog({
 
 							<div className="share-dialog-footer">
 								<button className="ghost" onClick={onClose}>
-									{t("common.cancel")}
+									{t("common.close")}
 								</button>
 								<button onClick={onGenerateShareLink} disabled={shareGenerating || !shareSelectedPerson}>
 									{shareGenerating ? t("share.generating") : t("share.generateLink")}