fix: restore let-chains and add rust-version = 1.87 to Cargo.toml

Let-chains require Rust 1.87+ (stabilized May 2025). Added rust-version
field to document the MSRV so cargo warns on older toolchains.

src/dashboard/websocket.rs

@@ -65,7 +65,7 @@ pub(super) async fn handle_websocket_connection(mut socket: WebSocket, state: Da
 pub(super) async fn handle_websocket_message(text: &str, state: &DashboardState) {
     // Parse and handle WebSocket messages
     if let Ok(data) = serde_json::from_str::<serde_json::Value>(text)
-        && let Some("ping") = data.get("type").and_then(|v| v.as_str())
+        && data.get("type").and_then(|v| v.as_str()) == Some("ping")
     {
         let _ = state.app_state.dashboard_tx.send(serde_json::json!({
             "type": "pong",