Move mumble UDP ping into common crate (#33)

The desktop GUI doesn't go through the proxy to reach the Mumble server,
so its login screen needs to ping directly. Rather than duplicate the
ping logic, move it into the common crate behind an optional
`networking` feature (so common stays lightweight when the feature
isn't requested).

- common: add `ping_server(address, port)` behind `networking` feature
- proxy: replace the inline UdpSocket ping + ping.rs codec with a call
  to common::ping_server; drop the rand dep
- client: PlatformInterface::get_status now takes an address; desktop
  and mobile call common::ping_server directly, web still uses the
  proxy's /status endpoint
- gui: thread the address from the login input through get_status, so
  it re-pings when the user edits the address

Assisted-by: claude-opus-4-7
Reviewed-on: #33
Reviewed-by: restitux <restitux@ohea.xyz>
Co-authored-by: Sam Sartor <me@samsartor.com>
Co-committed-by: Sam Sartor <me@samsartor.com>

client/src/imp/connect.rs

@@ -1,5 +1,5 @@
 use crate::app::{Command, SharedState};
-use color_eyre::eyre::{bail, Error};
+use color_eyre::eyre::Error;
 use futures_channel::mpsc::UnboundedReceiver;
 use mumble_protocol::control::ClientControlCodec;
 use std::net::ToSocketAddrs;
@@ -14,7 +14,7 @@ use tokio_rustls::TlsConnector;
 use tokio_util::compat::{TokioAsyncReadCompatExt as _, TokioAsyncWriteCompatExt as _};
 use tracing::{info, instrument};
 
-use mumble_web2_common::{ProxyOverrides, ServerStatus};
+use mumble_web2_common::ProxyOverrides;
 
 #[derive(Debug)]
 struct NoCertificateVerification;
@@ -108,10 +108,6 @@ pub async fn network_connect(
     crate::network_loop(username, state, event_rx, outgoing_send, reader).await
 }
 
-pub async fn get_status(client: &reqwest::Client) -> color_eyre::Result<ServerStatus> {
-    bail!("status not supported on desktop yet")
-}
-
 #[allow(unused)]
 pub use tokio::spawn;
 #[allow(unused)]