Add ServerEntry model and server list persistence to platform trait

Introduces a ServerEntry struct in common with name, address, port,
username, and optional password fields. Extends PlatformInterface with
load_servers/save_servers methods, implemented across all platforms
(desktop persists to JSON config, web uses localStorage, mobile/stub
are stubs).

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>,
+}