Previously, the API endpoint was defined using `localhost`, which could resolve to the IPv6 address `::1`.
This caused issues because the Portmaster core service is listening on `127.0.0.1:817` and only fast-tracking connections to that specific IPv4 address.
Replaced the http-client plugin, as it does not support keep-alive connections.
Each request opened a new TCP connection to the service, which was inefficient.
The new custom `send_tauri_http_request` command, exposed to the UI, uses an application-wide `reqwest::Client`, which supports idle (persistent) connections.
