Add server list persistence and UDP ping to platform trait

Add ServerEntry model, load_servers/save_servers/set_default_server
to PlatformInterface with implementations for desktop (etcetera config),
web (localStorage), mobile (stub), and stub platforms. Implement
mumble_udp_ping protocol on desktop for direct server status queries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/src/imp/stub.rs

@@ -3,7 +3,7 @@
 use crate::{app::SharedState, effects::AudioProcessor};
 use color_eyre::eyre::Error;
 use dioxus::hooks::UnboundedReceiver;
-use mumble_web2_common::{ProxyOverrides, ServerStatus};
+use mumble_web2_common::{ProxyOverrides, ServerEntry, ServerStatus};
 use std::future::Future;
 
 pub struct StubPlatform;
@@ -36,10 +36,29 @@ impl super::PlatformInterface for StubPlatform {
         async { panic!("stubbed platform") }
     }
 
+    fn ping_server(
+        _address: &str,
+        _port: u16,
+    ) -> impl Future<Output = color_eyre::Result<ServerStatus>> {
+        async { panic!("stubbed platform") }
+    }
+
     fn load_proxy_overrides() -> impl Future<Output = color_eyre::Result<ProxyOverrides>> {
         async { panic!("stubbed platform") }
     }
 
+    fn set_default_server(_server: &str) -> Option<()> {
+        panic!("stubbed platform")
+    }
+
+    fn load_servers() -> Vec<ServerEntry> {
+        panic!("stubbed platform")
+    }
+
+    fn save_servers(_servers: &[ServerEntry]) {
+        panic!("stubbed platform")
+    }
+
     fn sleep(_duration: std::time::Duration) -> impl Future<Output = ()> {
         async { panic!("stubbed platform") }
     }