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>

util/util.go

@@ -491,7 +491,7 @@ func ValidateAndFixSubnetRanges(db store.IStore) error {
 		return nil
 	}
 
-	server, err := db.GetServer()
+	server, err := db.GetServerByID(DefaultServerID)
 	if err != nil {
 		return err
 	}
@@ -639,7 +639,7 @@ func SendRequestedConfigsToTelegram(db store.IStore, userid int64) []string {
 			}
 
 			// build config
-			server, _ := db.GetServer()
+			server, _ := db.GetServerByID(DefaultServerID)
 			globalSettings, _ := db.GetGlobalSettings()
 			config := BuildClientConfig(*clientData.Client, server, globalSettings)
 			configData := []byte(config)