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>

router/router.go

@@ -81,11 +81,6 @@ func New(tmplDir fs.FS, extraData map[string]interface{}, secret [64]byte) *echo
 		log.Fatal(err)
 	}
 
-	tmplServerString, err := util.StringFromEmbedFile(tmplDir, "server.html")
-	if err != nil {
-		log.Fatal(err)
-	}
-
 	tmplGlobalSettingsString, err := util.StringFromEmbedFile(tmplDir, "global_settings.html")
 	if err != nil {
 		log.Fatal(err)
@@ -129,7 +124,6 @@ func New(tmplDir fs.FS, extraData map[string]interface{}, secret [64]byte) *echo
 	templates["login.html"] = template.Must(template.New("login").Funcs(funcs).Parse(tmplLoginString))
 	templates["profile.html"] = template.Must(template.New("profile").Funcs(funcs).Parse(tmplBaseString + tmplProfileString))
 	templates["clients.html"] = template.Must(template.New("clients").Funcs(funcs).Parse(tmplBaseString + tmplClientsString))
-	templates["server.html"] = template.Must(template.New("server").Funcs(funcs).Parse(tmplBaseString + tmplServerString))
 	templates["global_settings.html"] = template.Must(template.New("global_settings").Funcs(funcs).Parse(tmplBaseString + tmplGlobalSettingsString))
 	templates["users_settings.html"] = template.Must(template.New("users_settings").Funcs(funcs).Parse(tmplBaseString + tmplUsersSettingsString))
 	templates["status.html"] = template.Must(template.New("status").Funcs(funcs).Parse(tmplBaseString + tmplStatusString))