feat(ui): icon-only header actions and side-nav separators

- Help and Sign in/out become icon-only Carbon cds-header-global-action
  items (real links, accessible name in a visually-hidden span); the
  header nav keeps just Home + the tools.
- Side nav gains a separator so Home, the tools, and Help/Sign In/Out
  form three groups.
- Drop the now-unused header-nav-divider rule.

tools/public/assets/style.css

@@ -68,9 +68,6 @@ cds-side-nav[expanded] { pointer-events: auto; }
 .side-nav-brand { display: flex; align-items: center; gap: 0.75rem; padding: 2rem 1rem 1rem; color: var(--cds-text-primary, #161616); font-weight: 600; font-size: 14px; }
 .side-nav-brand img { display: block; }
 
-/* separator between the service links and Help in the header nav */
-.header-nav-divider { width: 1px; align-self: stretch; margin: 0.7rem 0.25rem; background: var(--cds-border-subtle, #e0e0e0); }
-
 /* ── layout ─────────────────────────────────────────────────────────────── */
 .container { width: 100%; max-width: 1136px; margin: 0 auto; padding: 0 1.5rem; }
 main.container { flex: 1 0 auto; padding-top: 1.5rem; padding-bottom: 1.5rem; }