refactor(enhance): simplify tun mode check and lowercase helpers

- Drop the always-true second clause in use_tun's enhanced-mode check:
  the key is absent only when the unwrap_or default already makes
  current_mode == "fake-ip" true.
- Collapse use_lowercase into a delegation to use_lowercase_owned.

src-tauri/src/enhance/field.rs

@@ -25,14 +25,7 @@ fn lowercase_key(key: &str) -> Value {
 }
 
 pub fn use_lowercase(config: &Mapping) -> Mapping {
-    let mut lowercased = Mapping::new();
-
-    for (key, value) in config.into_iter() {
-        if let Some(key_str) = key.as_str() {
-            lowercased.insert(lowercase_key(key_str), value.clone());
-        }
-    }
-    lowercased
+    use_lowercase_owned(config.clone())
 }
 
 pub fn use_lowercase_owned(config: Mapping) -> Mapping {

src-tauri/src/enhance/tun.rs

@@ -28,7 +28,7 @@ pub fn use_tun(mut config: Mapping, enable: bool) -> Mapping {
             .and_then(|v| v.as_str())
             .unwrap_or("fake-ip");
 
-        if current_mode == "fake-ip" || !dns_val.contains_key(Value::from("enhanced-mode")) {
+        if current_mode == "fake-ip" {
             revise!(dns_val, "enable", true);
             revise!(dns_val, "ipv6", ipv6_val);