Remove legacy default-server page, unify on multi-server registry

Deletes /wg-server (route, handlers, template) entirely and adds generic
/servers/:id/interface and /servers/:id/keypair endpoints + UI in the
All Servers page, so every server (including the default one) is managed
through the same per-server registry. Drops the write-through dual-write
hacks that kept the old single-server collection in sync - the registry
is now the single source of truth. One-time legacy-install migration path
is untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/base.html

@@ -128,16 +128,6 @@
                         </li>
 
                         {{if .baseData.Admin}}
-                        <li class="nav-item">
-                            <a href="{{.basePath}}/wg-server" class="nav-link {{if eq .baseData.Active "wg-server" }}active{{end}}">
-                                <i class="nav-icon fas fa-server"></i>
-                                <p>
-                                    Default Server Interface
-                                </p>
-                            </a>
-                        </li>
-
-
                         <li class="nav-header">SETTINGS</li>
                         <li class="nav-item">
                             <a href="{{.basePath}}/servers-settings" class="nav-link {{if eq .baseData.Active "servers" }}active{{end}}">