Clarify server nav labels (Default Server Interface vs All Servers)

No route/logic changes - pure label cleanup so the legacy default-server
editor and the multi-server management page are no longer confusable.

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

templates/server.html

@@ -1,5 +1,5 @@
 {{define "title"}}
-Wireguard Server
+Default Server Interface
 {{end}}
 
 {{define "top_css"}}
@@ -10,13 +10,13 @@ Wireguard Server
 {{end}}
 
 {{define "page_title"}}
-Wireguard Server Settings
+Default Server Interface Settings
 {{end}}
 
 {{define "page_content"}}
 <section class="content">
     <div class="container-fluid">
-        <!-- <h5 class="mt-4 mb-2">Wireguard Server</h5> -->
+        <!-- <h5 class="mt-4 mb-2">Default Server Interface</h5> -->
         <div class="row">
             <!-- left column -->
             <div class="col-md-6">