Fix "no such file" error creating clients on non-default servers

SuggestIPAllocation always looked up the legacy "wg0" server regardless
of which server the request was for, so any setup without a migrated
wg0 (i.e. every fresh multi-server install) failed with
"open db/servers/wg0.json: no such file or directory" when adding a
new client.

Now accepts an optional server_id query param (falling back to wg0 for
the legacy bare routes), and the per-server clients page passes its
own server ID.

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

handler/routes.go

@@ -1588,7 +1588,11 @@ func GetOrderedSubnetRanges() echo.HandlerFunc {
 // SuggestIPAllocation handler to get the list of ip address for client
 func SuggestIPAllocation(db store.IStore) echo.HandlerFunc {
 	return func(c echo.Context) error {
-		server, err := db.GetServerByID(util.DefaultServerID)
+		serverID := c.QueryParam("server_id")
+		if serverID == "" {
+			serverID = util.DefaultServerID
+		}
+		server, err := db.GetServerByID(serverID)
 		if err != nil {
 			log.Error("Cannot fetch server config from database: ", err)
 			return c.JSON(http.StatusBadRequest, jsonHTTPResponse{false, err.Error()})