fix: HTML sitemap page 1 redirects with the locale's segment

- seoFirstPageRedirect answers the unnumbered URL or null when the target
  is the URL itself, so /mapa-strony/<group>/strona/1 no longer loops (L4 WR-01)
- SeoHtmlSitemapPage takes the segment from the site answer, else the build
  option, else the param, and redirects only when the helper answers a target

app/components/global/SeoHtmlSitemapPage.vue

@@ -11,7 +11,7 @@
 // Registered globally under the stable name SeoHtmlSitemapPage: an app
 // overrides it by registering its own component of that name.
 import { formatSeoLabel } from '../../utils/labels.ts'
-import { seoPageHref, seoPageNumber } from '../../utils/pageHref.ts'
+import { seoFirstPageRedirect, seoPageNumber } from '../../utils/pageHref.ts'
 import { seoSitePath } from '../../utils/sitePath.ts'
 import type { SeoBreadcrumb, SeoSitemapGroupPage, SeoSitemapOverview } from '../../utils/types.ts'
 
@@ -25,16 +25,21 @@ const labels = useSeoLabels(() => props.locale)
 const own = (config.public.seo as { htmlSitemap?: { paths?: Record<string, string> } | false } | undefined)?.htmlSitemap
 const basePath = (own && typeof own === 'object' ? own.paths?.[props.locale] : '') || route.path
 
-// The page number of a group page, in the site's pagination form.
-const form = site.value?.pagination?.form === 'path' ? 'path' : 'query'
+// The page number of a group page: from the path-form route's :page, else
+// from the query parameter.
 const param = site.value?.pagination?.param || 'page'
+const ownPagination = (config.public.seo as { pagination?: { segments?: Record<string, string> } } | undefined)?.pagination
+// The locale's path segment, as the path-form route was registered (L4 WR-01).
+const segment = site.value?.pagination?.segments?.[props.locale] || ownPagination?.segments?.[props.locale] || param
 let page = 1
 if (props.group) {
   const parsed = props.pathPage ? (/^\d{1,6}$/.test(props.pathPage) ? Number(props.pathPage) : null) : seoPageNumber(route.fullPath, 'query', '', param)
   if (parsed === null || parsed < 1) throw createError({ statusCode: 404, statusMessage: 'Page Not Found', fatal: true })
   page = parsed
   const explicitFirst = props.pathPage === '1' || route.query[param] === '1'
-  if (explicitFirst) await navigateTo(seoPageHref(route.fullPath, 1, form, '', param), { redirectCode: 301 })
+  // A 301 to the unnumbered URL, never to the page's own URL.
+  const target = explicitFirst ? seoFirstPageRedirect(route.fullPath, props.pathPage ? 'path' : 'query', segment, param) : null
+  if (target) await navigateTo(target, { redirectCode: 301 })
 }
 
 const { data, error } = await useFetch<SeoSitemapOverview | SeoSitemapGroupPage>('/api/__seo__/sitemap', {

app/utils/pageHref.ts

@@ -29,6 +29,17 @@ export function seoPageHref(url: string, page: number, form: SeoPaginationForm =
   return qs ? `${path}?${qs}` : path
 }
 
+/**
+ * Where an explicit page 1 (/mapa-strony/posty/strona/1, /blog?page=1)
+ * redirects: the unnumbered URL, built with the locale's segment, or null
+ * when that target is the URL itself (a segment the URL does not carry), so
+ * a page never answers a 301 to its own address (L4 WR-01).
+ */
+export function seoFirstPageRedirect(fullPath: string, form: SeoPaginationForm, segment = '', param = 'page'): string | null {
+  const target = seoPageHref(fullPath, 1, form, segment, param)
+  return target === fullPath ? null : target
+}
+
 /**
  * The page number a URL carries in the given form: 1 without one, the
  * number for a positive integer, null for anything else (a 404).

tests/pageHref.test.ts

@@ -1,6 +1,6 @@
 import assert from 'node:assert/strict'
 import test from 'node:test'
-import { seoPageHref, seoPageNumber } from '../app/utils/pageHref.ts'
+import { seoFirstPageRedirect, seoPageHref, seoPageNumber } from '../app/utils/pageHref.ts'
 import { seoFillPages } from '../app/utils/seo.ts'
 
 test('seoPageHref: the UI-SPEC L2 examples', () => {
@@ -45,3 +45,17 @@ test('seoFillPages: %pages% in the title and meta, only once the count is known'
   assert.equal(seoFillPages(head, null), head)
   assert.equal(seoFillPages(head, 0), head)
 })
+
+test('seoFirstPageRedirect: page 1 goes to the unnumbered URL with the locale segment (L4 WR-01)', () => {
+  assert.equal(seoFirstPageRedirect('/mapa-strony/posty/strona/1', 'path', 'strona', 'page'), '/mapa-strony/posty')
+  assert.equal(seoFirstPageRedirect('/sitemap/posts/page/1', 'path', 'page', 'page'), '/sitemap/posts')
+  assert.equal(seoFirstPageRedirect('/mapa-strony/posty?page=1', 'query', '', 'page'), '/mapa-strony/posty')
+  assert.equal(seoFirstPageRedirect('/mapa-strony/posty/strona/1?x=1', 'path', 'strona', 'page'), '/mapa-strony/posty?x=1')
+})
+
+test('seoFirstPageRedirect: never a redirect to the URL itself (no 301 loop)', () => {
+  // the segment missing: seoPageHref falls back to the param, which the URL does not carry
+  assert.equal(seoFirstPageRedirect('/mapa-strony/posty/strona/1', 'path', '', 'page'), null)
+  assert.equal(seoFirstPageRedirect('/mapa-strony/posty', 'query', '', 'page'), null)
+  assert.equal(seoFirstPageRedirect('/sitemap/posts', 'path', 'page', 'page'), null)
+})