[desktop] Use custom Tauri command for HTTP requests instead of http-client plugin

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.

desktop/tauri/src-tauri/Cargo.toml

@@ -25,7 +25,6 @@ tauri-plugin-single-instance = "2.2.1"
 tauri-plugin-notification = "2.2.1"
 tauri-plugin-log = "2.2.1"
 tauri-plugin-window-state = "2.2.1"
-tauri-plugin-http = "2"
 tauri-plugin-websocket = "2"
 
 clap_lex = "0.7.2"
@@ -49,7 +48,7 @@ http = "1.0.0"
 url = "2.5.0"
 thiserror = "1.0"
 log = "0.4.21"
-reqwest = { version = "0.12" }
+reqwest = { version = "0.12", features = ["cookies", "json"] }
 
 rfd = { version = "*", default-features = false, features = [ "tokio", "gtk3", "common-controls-v6" ] }
 open = "5.1.3"