Lang settings tab: flag-grid language picker, bootstrap es+zh targets

- New editor-shell tab (lang): translatable languages as clickable flag
  tiles (shared langs.js mapping of the 28 Seed-X languages to flags),
  saved via the settings round-trip; translator service URL(s) shown.
- Bootstrap seeds translate_langs with Spanish and Chinese (the original
  language is never a target).
- GET /_api/settings also returns primary_lang.
- PageEditor's flag/name helpers move to the shared langs.js.

frontend/src/EditorShell.vue

@@ -7,6 +7,7 @@ import PageEditor from './PageEditor.vue'
 import BannerEditor from './BannerEditor.vue'
 import SiteEditor from './SiteEditor.vue'
 import StructureEditor from './StructureEditor.vue'
+import LocalizationEditor from './LocalizationEditor.vue'
 
 const props = defineProps({
   pagePath: { type: String, default: '' },
@@ -17,11 +18,12 @@ const emit = defineEmits(['close'])
 const currentPath = ref(props.pagePath)
 const activeMode = ref(props.initialMode)
 
-// Tab order: site-wide settings first (site, structure), then — after a
-// visual break — the per-page editors (article, banner).
+// Tab order: site-wide settings first (site, structure, localization), then
+// — after a visual break — the per-page editors (article, banner).
 const MODES = [
   { key: 'site', label: 'site', component: SiteEditor },
   { key: 'structure', label: 'structure', component: StructureEditor },
+  { key: 'localization', label: 'lang', component: LocalizationEditor },
   { key: 'page', label: 'article', component: PageEditor, breakBefore: true },
   { key: 'banner', label: 'banner', component: BannerEditor },
 ]