Cap content-area height on every page, fix scroll bugs, sidebar accordion

- Apply the taskbar's max-height/overflow-y:auto treatment to
  #wpbody-content unconditionally, so tall native/Aurora pages (Add
  Plugins, Add User, Profile, Fonts, file editors, Settings, Activity Log,
  Database Explorer, Comments) scroll internally within their frame
  instead of growing past the viewport and scrolling the whole document.
- Fix a Chrome scroll-anchoring bug this surfaced on Add Plugins (page
  loaded scrolled ~1700px into content instead of at the top) via
  overflow-anchor: none.
- Fix a phantom document-level scrollbar with dead space below the frame
  by locking html/body's own overflow, since the sidebar/toolbar are fixed
  and content is only ever meant to scroll internally.
- Make the sidebar's native-menu sections a single-open accordion:
  expanding one now collapses whichever else was open.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/src/shell/components/Sidebar.vue

@@ -63,21 +63,16 @@ const isSectionActive = (item) => {
   return (item.submenu || []).some((sub) => normalize(sub.url) === current.value);
 };
 
-// Which sections are manually expanded. Active section starts expanded.
-const expanded = ref(new Set());
+// Accordion: only one section's submenu open at a time, so expanding one
+// closes the rest instead of the list growing indefinitely. Starts on
+// whichever section is active (each nav is a full page load in this
+// multi-page admin, so there's no need to react to URL changes afterward).
+const openId = ref(props.menu.find((item) => isSectionActive(item))?.id ?? null);
 
-const isExpanded = (item) => {
-  return expanded.value.has(item.id) || isSectionActive(item);
-};
+const isExpanded = (item) => openId.value === item.id;
 
 const toggle = (item) => {
-  const next = new Set(expanded.value);
-  if (next.has(item.id)) {
-    next.delete(item.id);
-  } else {
-    next.add(item.id);
-  }
-  expanded.value = next;
+  openId.value = openId.value === item.id ? null : item.id;
 };
 
 // Aurora's own branded nav section (Settings, Menu Creator, Admin Notices,