Dokan Pro <= 5.0.4 - Authenticated (Vendor+) Privilege Escalation via update_capabilities REST Endpoint

The Dokan Pro plugin for WordPress is vulnerable to privilege escalation via update_capabilities REST Endpoint in all versions up to, and including, 5.0.4. This is due to the `update_capabilities()` REST handler accepting arbitrary capability strings from the request body and passing them directly to WP_User::add_cap() with no allowlist validation, only verifying that the caller holds the dokandar capability. This makes it possible for authenticated attackers with a self-provisioned Vendor-level access and above, on sites with the Vendor Staff module enabled, to grant arbitrary WordPress capabilities, including administrator, to any vendor_staff account, leading to a full site takeover.