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>

common/src/lib.rs

@@ -16,3 +16,13 @@ pub struct ServerStatus {
     pub max_users: Option<u32>,
     pub bandwidth: Option<u32>,
 }
+
+#[derive(Debug, Clone, Deserialize, Serialize, Default, PartialEq)]
+pub struct ServerEntry {
+    pub name: String,
+    pub address: String,
+    pub port: u16,
+    pub username: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub password: Option<String>,
+}