Unify admin editors into a tabbed EditorShell (site, structure, article, banner)

- One mounted shell with four kept-alive tabs; pens open/switch tabs,
  closing only hides the shell so unsaved page-editor state survives
  until a real reload; admin panels never reload the page, regions are
  refreshed in place via the shared swapdoc.js helper
- SiteEditor split: site settings vs. the new StructureEditor tree tab;
  BannerEditor split out of the old site editor
- Article editor format bar (bold/italic/code/link/table/image) with a
  table size picker and Ctrl/Cmd-B/I/S bindings; fenced code block on an
  empty line; icon-only save button disabled (saturate(0)) when clean
- Media uploads use icon buttons; favicon upload by clicking the preview
  tile; placeholders reserved for actual defaults in effect
- Banner/site pens and auth buttons grouped in an .editor-pens container;
  page pen no longer shown to anonymous visitors
- While editing, window scroll is locked, the panel exactly fills the
  available window height, and only #main scrolls; editor scroll drives
  the article
- Banner design inherit label names the design and its true source;
  backend banner_design_source excludes the node's own setting and the
  doc payload carries banner_design_inherited

frontend/src/StructureTree.vue

@@ -1,6 +1,6 @@
 <script setup>
 // Recursive site-structure tree with drag-and-drop ordering (vue-draggable).
-// Nodes come from the server (GET /_api/pages via SiteEditor.vue) as
+// Nodes come from the server (GET /_api/pages via StructureEditor.vue) as
 // {slug, path, title, order, published, has_content, children}.
 // Every node is real: a label whose title and slug are always editable
 // inline — the title saves while typing (and focusing it opens the page),
@@ -19,8 +19,8 @@
 // The front page is the root row with an empty slug: renaming it away
 // leaves no front page, and giving another top-level row the empty slug
 // makes it the front page. Delete is a two-step inline button (no dialog).
-// Actions are injected from SiteEditor.vue to avoid per-level event
-// forwarding.
+// Actions are injected from the parent editor (StructureEditor.vue) to
+// avoid per-level event forwarding.
 import { inject } from 'vue'
 import draggable from 'vuedraggable'
 import { slugify } from './slugify'
@@ -104,7 +104,7 @@ function onEnd() {
               v-model="element.title"
               v-focus
               class="edit title-edit"
-              placeholder="Title"
+              title="Page title"
               @keyup.enter="handlers.commitPending()"
               @keyup.esc="handlers.discardPending()"
             />
@@ -126,7 +126,6 @@ function onEnd() {
             <input
               class="edit title-edit"
               :value="element.title"
-              placeholder="Title"
               title="Label in the navigation — saves while typing; click opens the page"
               @input="handlers.titleInput(element, $event)"
               @focus="handlers.open(element.path)"