fix: resolve 3 billing admin errors and normalize remote_servers query

- admin_config.php: guard session_start() — was firing Notice because
  admin_auth.php → session_bridge.php already started the session
- includes/menu.php: check mysqli_thread_id() before reusing $db so a
  closed handle does not cause 'mysqli object is already closed' fatal
- admin_invoices.php / admin_payments.php: set $db = null after
  mysqli_close() so menu.php's reuse-check correctly falls through to
  opening a fresh connection
- adminserverlist.php: use col_exists() to detect missing 'enabled'
  column in gsp_remote_servers; fall back to constant 1 and display a
  schema-notice banner; guard UPDATE accordingly; also add missing
  price_daily / price_year columns to the services SELECT; remove
  duplicate 'Update Enabled Servers' button
- add_remote_server_enabled_column.sql: idempotent migration to add the
  'enabled' INT column to gsp_remote_servers on older installs

Agent-Logs-Url: https://github.com/GameServerPanel/GSP/sessions/988997ed-7568-48bf-96ef-889fb5d91fec

Co-authored-by: iaretechnician <2749183+iaretechnician@users.noreply.github.com>

modules/billing/includes/menu.php

@@ -55,7 +55,9 @@ if ($is_logged_in) {
   // Prefer reusing an existing $db if present, otherwise open a local connection
   $menu_db = null;
   $menu_db_opened = false;
-  if (isset($db) && $db instanceof mysqli) {
+  // Only reuse $db if it is still an open (non-closed) connection.
+  // mysqli_thread_id() returns 0 on a closed handle, so guard against that.
+  if (isset($db) && $db instanceof mysqli && @mysqli_thread_id($db)) {
     $menu_db = $db;
   } else {
     $menu_db_port = isset($db_port) ? (int)$db_port : null;