fix(nav,footer): hide Activate from sidebar, surface it in Help popover

- Collapse the Account section: Activate now lives in the same
  hidden sidebar section as Close (single ``_hidden`` group). Both
  pages stay registered with ``st.navigation`` so /activate and
  /close remain URL-routable for the Help-popover / Close-button
  links — only the sidebar entries + their section header are
  hidden via CSS.
- Help popover always exposes a license-management link now:
  ``Activate now →`` when the license is inactive, ``Manage
  license →`` when it is active and valid. Both point at
  ``./activate``.
- Extend the sidebar-hide CSS to also match ``a[href$="/activate"]``
  and the section that contains it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/i18n/packs/en.json

@@ -183,6 +183,7 @@
     "help_license_active": "{name}",
     "help_license_expires": "Expires {date} ({days} days)",
     "help_activate_link": "Activate now →",
+    "help_manage_link": "Manage license →",
     "help_dismiss": "Close"
   }
 }