Read only_db restriction from a dedicated session key

da_login.php now stores the ticket's restrict_db into
DA_MYSQL_PHPMYADMIN_RESTRICT_DB, and config.inc.php's only_db builder
reads that key instead of the landing-page db field, so an
all-databases login (empty restrict_db) leaves phpMyAdmin unrestricted.

scripts/setup/phpmyadmin_install.sh

@@ -353,8 +353,7 @@ function da_mysql_phpmyadmin_only_db(): string
     session_name($sessionName);
     session_id($sessionId);
     session_start();
-    $auth = $_SESSION['DA_MYSQL_PHPMYADMIN_AUTH'] ?? [];
-    $db = is_array($auth) ? (string)($auth['db'] ?? '') : '';
+    $db = (string)($_SESSION['DA_MYSQL_PHPMYADMIN_RESTRICT_DB'] ?? '');
     session_write_close();
 
     return $db;
@@ -515,6 +514,7 @@ session_set_cookie_params([
 ]);
 session_start();
 $_SESSION['DA_MYSQL_PHPMYADMIN_AUTH'] = $ticket['auth'];
+$_SESSION['DA_MYSQL_PHPMYADMIN_RESTRICT_DB'] = (string)($ticket['restrict_db'] ?? '');
 session_write_close();
 
 header('Cache-Control: no-store, no-cache, must-revalidate');