Fix build errors: FTPS AsyncRead bridge, TLS connector type, unsafe env::set_var, dead code

Verified with cargo check + cargo run --example printer_polymorphism.

Cargo.toml

@@ -20,12 +20,11 @@ path = "examples/printer_polymorphism.rs"
 [dependencies]
 tokio = { version = "1.40", features = ["full"] }
 tokio-tungstenite = { version = "0.24", features = ["native-tls"] }
-tokio-util = { version = "0.7", features = ["codec"] }
+tokio-util = { version = "0.7", features = ["codec", "compat"] }
 futures-util = "0.3"
 rumqttc = "0.24"
 reqwest = { version = "0.12", features = ["stream", "json"] }
 suppaftp = { version = "6", features = ["async-native-tls"] }
-native-tls = "0.2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 rusqlite = { version = "0.32", features = ["bundled"] }
@@ -36,6 +35,7 @@ anyhow = "1"
 thiserror = "1"
 uuid = { version = "1", features = ["v4", "serde"] }
 rand = "0.8"
+dotenvy = "0.15"
 
 # continuum-common's rust-core crate (error types, tracing init, G-code helpers)
 # lives in the sibling `continuum-common` repo. Once that crate is published to