fix: retry qr code login polling for known transient gateway responses

frontend/src/routes/login/alternative/device/+page.svelte

@@ -13,6 +13,7 @@
 	import userStore from '$lib/stores/user-store';
 	import type { DeviceLoginRequest } from '$lib/types/device-login.type';
 	import { getAxiosErrorMessage } from '$lib/utils/error-util';
+	import { isAxiosError } from 'axios';
 	import { mode } from 'mode-watcher';
 	import { onMount } from 'svelte';
 	import LoginLogoErrorSuccessIndicator from '../../components/login-logo-error-success-indicator.svelte';
@@ -37,6 +38,7 @@
 
 	async function startRequest() {
 		stopRequest();
+		// Track this attempt so stale responses cannot update the current page
 		const controller = new AbortController();
 		requestController = controller;
 		request = undefined;
@@ -45,12 +47,14 @@
 
 		try {
 			request = await deviceLoginService.createRequest(controller.signal);
+			// Ignore a response from an attempt replaced while the request was in flight
 			if (requestController !== controller) return;
 			schedulePoll(controller);
 		} catch (error) {
 			if (controller.signal.aborted) return;
 			errorMessage = getAxiosErrorMessage(error);
 		} finally {
+			// Stop the initial loading state only if this attempt still owns the page
 			if (requestController === controller) {
 				isStarting = false;
 			}
@@ -58,15 +62,19 @@
 	}
 
 	function schedulePoll(controller: AbortController) {
+		// Ignore schedules from stale attempts or before request metadata exists
 		if (!request || requestController !== controller) return;
+		// Delay the next exchange so the client respects the server-provided polling interval
 		pollTimer = setTimeout(() => void exchangeRequest(controller), request.interval * 1000);
 	}
 
 	async function exchangeRequest(controller: AbortController) {
+		// Ignore work from a stopped or superseded attempt
 		if (!request || requestController !== controller) return;
 
 		try {
 			const user = await deviceLoginService.exchangeRequest(request.id, controller.signal);
+			// Ignore a response from an attempt replaced while the exchange was in flight
 			if (requestController !== controller) return;
 			if (!user) {
 				schedulePoll(controller);
@@ -75,22 +83,42 @@
 
 			clearTimers();
 			await userStore.setUser(user);
+			// Navigate only after confirming that this attempt is still active
 			if (requestController !== controller) return;
 			await goto(data.redirect);
 		} catch (error) {
 			if (controller.signal.aborted) return;
+			// Retry transport and proxy failures because the device login request remains pending
+			if (isRetryableDeviceLoginExchangeError(error)) {
+				schedulePoll(controller);
+				return;
+			}
+			// Stop retrying when the server reports a non-transient failure
 			clearTimers();
 			errorMessage = getAxiosErrorMessage(error);
 		}
 	}
 
+	function isRetryableDeviceLoginExchangeError(error: unknown) {
+		// Ignore non-Axios errors and intentional cancellations
+		if (!isAxiosError(error) || error.code === 'ERR_CANCELED') return false;
+
+		const retryableExchangeStatuses = new Set([408, 425, 429, 502, 503, 504, 522, 524]);
+		// Retry failures without an HTTP response or known transient gateway responses
+		return (
+			error.response?.status === undefined || retryableExchangeStatuses.has(error.response.status)
+		);
+	}
+
 	function stopRequest() {
+		// Abort the active exchange and clear its timer before replacing or leaving it
 		requestController?.abort();
 		requestController = undefined;
 		clearTimers();
 	}
 
 	function clearTimers() {
+		// Ensure an old scheduled poll cannot start after the request lifecycle ends
 		if (pollTimer) clearTimeout(pollTimer);
 		pollTimer = undefined;
 	}