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/mod.rs

@@ -86,9 +86,14 @@ pub trait PlatformInterface {
         state: SharedState,
     ) -> impl Future<Output = Result<(), Error>>;
 
-    /// Get server status (user count, version, etc.).
+    /// Get server status (user count, version, etc.) for the given address.
+    ///
+    /// On web, this goes through the proxy's /status endpoint and ignores `address`
+    /// (the proxy is bound to a specific server). On desktop/mobile, this pings the
+    /// given address directly via UDP.
     fn get_status(
         client: &reqwest::Client,
+        address: &str,
     ) -> impl Future<Output = color_eyre::Result<ServerStatus>>;
 
     /// Load the proxy overrides (proxy URL, cert hash, etc.).