Nested sidebar nav, banner inherit label fix, editor UX polish

- Sidebar renders the section's whole subtree as nested lists (third
  level and deeper, article-list-style markers); the one-item sidebar
  rule yields when that item has children of its own; only the viewed
  page is highlighted (navbar keeps ancestor highlighting); first_leaf
  skips unpublished branches
- Banner design selector: inherit option names the resolved design and
  its true source (backend banner_design_source excludes the node's own
  setting; doc payload carries banner_design_inherited)
- Article editor: fenced code block on empty lines, table size picker
  popup, borderless save icon disabled by saturate(0) alone
- While editing, window scroll is locked and only #main scrolls; the
  panel exactly fills the available window height; editor scroll drives
  the article

frontend/src/assets/pagerite.css

@@ -354,6 +354,22 @@ body.editing #main {
   line-height: 1.0;
 }
 
+/* Third level (and deeper): nested like the first level of article lists —
+   slight indent, emoji marker — while the sidebar's own top level (the
+   section's direct children) stays plain, without indicators or indent. */
+#sidebar ul ul {
+  gap: 0.5em;
+  margin-top: 0.5em;
+  padding-inline-start: 1em;
+}
+
+#sidebar ul ul li::before {
+  content: "🔹";
+  display: inline-block;
+  margin-left: -1.3em;
+  width: 1.3em;
+}
+
 #sidebar a {
   text-decoration: none;
   color: var(--muted);