fix: Linux 系统安装时游戏目录默认回退到用户主目录

从 deb 等系统包安装时，启动器可执行文件位于 /usr/bin，此前一键安装
StarLight 实例的"选择游戏目录"弹窗会默认到这个系统目录，普通用户既
无写权限，也不适合存放游戏数据。

将默认目录计算移入 theseus::default_game_dir_root()：Linux 下若可执行
文件位于 /usr、/opt、/snap 等系统路径，回退到用户主目录；便携版与其
他平台仍使用可执行文件所在目录。

packages/app-lib/src/lib.rs

@@ -39,7 +39,7 @@ pub use state::{DirectoryInfo, State};
 pub use storage::*;
 pub use util::fetch::build_proxied_client;
 pub use util::file_lock::{LockingProcess, get_locking_processes};
-pub use util::platform::is_process_elevated;
+pub use util::platform::{default_game_dir_root, is_process_elevated};
 pub use util::proxy::{ProxyConfig, ProxyMode, ProxyTestResult};
 pub use util::symlink::SymlinkCapability;
 

packages/app-lib/src/util/platform.rs

@@ -1,5 +1,6 @@
 //! Platform-related code
 use daedalus::minecraft::{Os, OsRule};
+use std::path::PathBuf;
 
 /// (e.g. "Run as administrator" on Windows).
 #[cfg(target_os = "windows")]
@@ -143,3 +144,31 @@ pub fn classpath_separator(java_arch: &str) -> &'static str {
         Os::Windows | Os::WindowsArm64 => ";",
     }
 }
+
+/// Root directory used as the default for one-click StarLight instance
+/// installs. Portable builds live next to their executable, which is a fine
+/// place for game data. System installs (e.g. a Linux `.deb` under `/usr/bin`)
+/// do not, so fall back to the user's home directory there.
+pub fn default_game_dir_root() -> Option<PathBuf> {
+    let exe_dir = std::env::current_exe().ok()?.parent()?.to_path_buf();
+    if cfg!(target_os = "linux") && is_system_install_dir(&exe_dir) {
+        return dirs::home_dir();
+    }
+    Some(exe_dir)
+}
+
+#[cfg(target_os = "linux")]
+fn is_system_install_dir(path: &std::path::Path) -> bool {
+    const SYSTEM_PREFIXES: &[&str] = &[
+        "/usr/", "/opt/", "/snap/", "/bin/", "/sbin/", "/lib/", "/lib64/",
+    ];
+    let display = path.to_string_lossy();
+    SYSTEM_PREFIXES
+        .iter()
+        .any(|prefix| display.starts_with(prefix))
+}
+
+#[cfg(not(target_os = "linux"))]
+fn is_system_install_dir(_path: &std::path::Path) -> bool {
+    false
+}