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/onboarding.html

@@ -125,9 +125,8 @@
           <span class="onboarding-step-icon">🌐</span>
           <h2 class="onboarding-step-title">Domain Configuration</h2>
           <p class="onboarding-step-desc">
-            Sovran_SystemsOS uses <strong><a href="https://njal.la" target="_blank" style="color: var(--accent-color);">Njal.la</a></strong> for domains and Dynamic DNS.
-            Create an account at Njal.la, then for each service below, add a <strong>Dynamic</strong> record — no IP needed, it auto-populates once the DDNS curl command runs.
-            Paste the curl command from your Njal.la dashboard for each service.
+            Sovran_SystemsOS uses <strong><a href="https://njal.la" target="_blank" style="color: var(--accent-color);">Njal.la</a></strong> for domains and Dynamic DNS, and your router needs ports <strong>80</strong> and <strong>443</strong> (TCP) forwarded to this computer.
+            Everything you need — Njal.la account, Dynamic records, and the one router task — is laid out step by step below.
           </p>
         </div>
         <div class="onboarding-card" id="step-3-body">
@@ -170,6 +169,7 @@
     </div><!-- /panel-wrap -->
   </div><!-- /shell -->
 
+  <script src="/static/js/domain-prereqs.js?v={{ asset_version }}"></script>
   <script src="/static/onboarding.js?v={{ onboarding_js_hash }}"></script>
 </body>
 </html>