refactor: 补全 Default trait + 简化 Rust 2021 惯用写法

- LinuxArm64Backend: 添加 Default impl（与其他插件保持一致）
- version_manager.rs: [active, stable].into_iter() 替换 IntoIterator::into_iter()
  (Rust 2021 edition 原生支持数组 IntoIterator)

src/core/version_manager.rs

@@ -127,7 +127,8 @@ impl VersionManager {
 
         let active = entry.map(|e| e.active_version.as_str());
         let stable = entry.and_then(|e| e.last_stable_version.as_deref());
-        let protected_count = IntoIterator::into_iter([active, stable])
+        let protected_count = [active, stable]
+            .into_iter()
             .flatten()
             .collect::<HashSet<_>>()
             .len();

src/plugins/device/linux_arm64.rs

@@ -24,6 +24,12 @@ pub struct LinuxArm64Backend {
     backlight_enabled: bool,
 }
 
+impl Default for LinuxArm64Backend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl LinuxArm64Backend {
     /// 创建新的 Linux ARM64 后端实例
     pub fn new() -> Self {