Move mumble UDP ping into common crate

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

gui/src/main.rs

@@ -502,15 +502,6 @@ pub fn LoginView(overrides: Resource<ProxyOverrides>) -> Element {
     let user_config = use_context::<ConfigSystem>();
     let net: Coroutine<Command> = use_coroutine_handle();
 
-    let last_status = use_signal(|| None::<color_eyre::Result<ServerStatus>>);
-    use_resource(move || async move {
-        let client = reqwest::Client::new();
-        loop {
-            *last_status.write_unchecked() = Some(Platform::get_status(&client).await);
-            Platform::sleep(std::time::Duration::from_secs_f32(1.0)).await;
-        }
-    });
-
     let mut address_input = use_signal(|| user_config.config_get::<String>("server_url"));
     let address = use_memo(move || {
         if let Some(addr) = address_input() {
@@ -522,6 +513,19 @@ pub fn LoginView(overrides: Resource<ProxyOverrides>) -> Element {
         }
     });
 
+    let last_status = use_signal(|| None::<color_eyre::Result<ServerStatus>>);
+    use_resource(move || {
+        let addr = address();
+        async move {
+            let client = reqwest::Client::new();
+            loop {
+                *last_status.write_unchecked() =
+                    Some(Platform::get_status(&client, &addr).await);
+                Platform::sleep(std::time::Duration::from_secs_f32(1.0)).await;
+            }
+        }
+    });
+
     let mut username = use_signal(|| {
         user_config
             .config_get::<String>("username")
@@ -640,7 +644,7 @@ pub fn LoginView(overrides: Resource<ProxyOverrides>) -> Element {
                     Some(Err(_)) => rsx!(div {
                         class: "login_status is_error",
                         span {
-                            "Could not reach proxy server"
+                            "Could not reach server"
                         }
                     }),
                 }