Unify Njal.la + router port-forwarding guidance across onboarding and feature enable modals

Single source of truth (static/js/domain-prereqs.js) for the domain
prerequisite instructions so all three surfaces read identically:

- Server + Desktop first-boot onboarding wizard (step 3)
- Lightning Wallet Connections (NWC) enable modal (Node-only mode)
- BTCPay Server (web) enable modal (Node-only mode)

Both feature-enable flows share openDomainSetupModal(), which now drops
its role-branched intro and 'Option A/B' blocks in favor of the shared
renderers. All surfaces now consistently cover:

  1. A domain from Njal.la (account, subdomain-vs-separate-domain,
     Dynamic record with host-part-only Name field, auto-filled IP,
     DDNS curl command)
  2. Router access — forward ports 80 & 443 (TCP) to this computer's
     internal IP, once, for HTTPS/SSL (with CGNAT note)
  3. How to get Njal.la working, step by step

The reconfigure/troubleshooting modal gains the same router reminder,
since 'domain not reachable' is often the port forwarding rather than
DNS. domain-prereqs.js is loaded via asset_version cache busting and
ships automatically (installPhase copies the package wholesale).

Co-authored-by: arena-agent <297053741+arena-agent@users.noreply.github.com>

app/sovran_systemsos_web/templates/index.html

@@ -312,6 +312,7 @@
   <script src="/static/js/constants.js?v={{ asset_version }}"></script>
   <script src="/static/js/state.js?v={{ asset_version }}"></script>
   <script src="/static/js/helpers.js?v={{ asset_version }}"></script>
+  <script src="/static/js/domain-prereqs.js?v={{ asset_version }}"></script>
   <script src="/static/js/tiles.js?v={{ asset_version }}"></script>
   <script src="/static/js/service-detail.js?v={{ asset_version }}"></script>
   <script src="/static/js/support.js?v={{ asset_version }}"></script>