Sections across the top, and a choice between the two shells

The old web UI put everything in a sidebar, and a reskin of it would
still read as the old web UI. The modern shell splits navigation in two
instead: the layout switcher already in the top bar is tier one, and a
new bar under it carries that layout's own top-level items, each group
opening its children in a menu. Nothing on the left, so a queue table or
a dashboard gets the whole window.

- Management's nine entries and Account's eleven suit a menu bar; the
  bar measures its items once per layout and folds whatever doesn't fit
  into "More", recomputing on resize from the cached widths.
- Settings keeps the sidebar. Nineteen groups is a configuration browser,
  not a set of destinations, and a menu bar stops helping however wide
  the window; the threshold is on the count, not on the name, so a
  changed schema can't strand anyone.
- Which shell to use is the reader's, beside the theme: user menu,
  Layout, Modern or Legacy. Modern is the default, and the choice is
  remembered in the browser rather than with the account — settings.json
  is shared with the webmail, which has no notion of this.
- A phone is unchanged. The section bar is hidden below md and the
  sidebar stays as the slide-over behind the hamburger.
- The tree walking both shells need moves to lib/navTree.ts, so the
  sidebar and the section bar agree on what is visible, what is locked
  and what is active.

src/i18n/en.json

@@ -301,6 +301,14 @@
   "version": {
     "label": "INBUXA Admin {{version}}"
   },
+  "nav": {
+    "layoutLegacy": "Legacy",
+    "layoutLegacyHint": "The sidebar, as the old web UI had it.",
+    "layoutMenu": "Layout",
+    "layoutModern": "Modern",
+    "layoutModernHint": "Sections across the top; the page gets the full width.",
+    "more": "More"
+  },
   "oauth": {
     "backToLogin": "Back to login",
     "discoveryFailed": "Discovery failed for \"{{username}}\": {{status}} {{statusText}}",