Per-node primary language; home-country flags; grouped lang grid

- Node.language sets an article's primary language, inherited down the
  hierarchy, configured per row in the structure tab; English is now a
  regular language in the lang tab, still the root fallback
- Flags map a base tag to the language's home country (en → GB,
  pt → PT); explicit variant tags resolve their own region
- Lang tab flag grid: geographic group rows, shading-only selection

frontend/src/StructureTree.vue

@@ -1,7 +1,10 @@
 <script setup>
 // Recursive site-structure tree with drag-and-drop ordering (vue-draggable).
 // Nodes come from the server (GET /_api/pages via StructureEditor.vue) as
-// {slug, path, title, translated, order, published, has_content, children}.
+// {slug, path, title, translated, order, published, has_content, language,
+// primary, children}. The row's flag (LangSelect) sets the node's primary
+// language (language; '' = inherit — dimmed, showing the resolved flag);
+// the setting covers the whole subtree.
 // With a `lang` prop (StructureEditor's language strip) the titles shown
 // are that language's; `translated` marks rows with an actual translation
 // (untranslated rows show the original title, dimmed).
@@ -27,6 +30,7 @@
 import { inject } from 'vue'
 import draggable from 'vuedraggable'
 import { slugify } from './slugify'
+import LangSelect from './LangSelect.vue'
 
 defineOptions({ name: 'StructureTree' })
 const props = defineProps({
@@ -150,6 +154,17 @@ function onEnd() {
               @change="handlers.commitSlug(element, $event)"
             />
             <span class="acts">
+              <span
+                class="row-lang"
+                :class="{ inherited: !element.language }"
+              ><LangSelect
+                :model-value="element.language"
+                :options="handlers.langOptions(element)"
+                :title="element.language
+                  ? `primary language: set on this page (subtree inherits)`
+                  : `primary language: inherited — set it here (subtree inherits)`"
+                @update:model-value="handlers.setLanguage(element, $event)"
+              /></span>
               <span v-if="!element.published" class="draft">draft</span>
               <button
                 v-if="element.has_content || !element.children.length"
@@ -234,7 +249,7 @@ body.tree-dragging .treelist {
    level, not across levels). */
 .row {
   display: grid;
-  grid-template-columns: 1.2em minmax(3rem, 1fr) 7rem 5rem;
+  grid-template-columns: 1.2em minmax(3rem, 1fr) 7rem auto;
   align-items: baseline;
   gap: 0.35rem;
   /* Vertical spacing widens the drop zones: the exposed top strip is the
@@ -303,6 +318,20 @@ body.tree-dragging .treelist {
   justify-content: end;
 }
 
+/* Row language selector (LangSelect): the effective primary language's
+   flag; dimmed while the setting is inherited rather than set on the row. */
+.row-lang {
+  display: inline-flex;
+}
+
+.row-lang.inherited :deep(.lang-current) {
+  opacity: 0.45;
+}
+
+.row-lang.inherited:hover :deep(.lang-current) {
+  opacity: 0.85;
+}
+
 .draft {
   color: var(--muted);
   font-size: 0.75rem;