fix(env): gate PowerShell/cmd.exe shell detection to Windows only

`detect_shell()` checked for `PSModulePath` to detect PowerShell, but
on macOS this variable can be set by Homebrew pwsh even in bash/zsh,
causing `vp env use` to output PowerShell-style commands. Gate both
`PSModulePath` and `COMSPEC` checks behind `cfg!(windows)` so
non-Windows shells default to POSIX after ruling out fish.

Author: fengmk2

crates/vite_global_cli/src/commands/env/use.rs

@@ -31,9 +31,9 @@ enum Shell {
 fn detect_shell() -> Shell {
     if std::env::var("FISH_VERSION").is_ok() {
         Shell::Fish
-    } else if std::env::var("PSModulePath").is_ok() {
+    } else if cfg!(windows) && std::env::var("PSModulePath").is_ok() {
         Shell::PowerShell
-    } else if std::env::var("COMSPEC").is_ok() {
+    } else if cfg!(windows) {
         Shell::Cmd
     } else {
         Shell::Posix
@@ -136,8 +136,59 @@ pub async fn execute(
 
 #[cfg(test)]
 mod tests {
+    use serial_test::serial;
+
     use super::*;
 
+    #[test]
+    #[serial]
+    fn test_detect_shell_posix_even_with_psmodulepath() {
+        // SAFETY: This test runs in isolation with serial_test
+        unsafe {
+            std::env::remove_var("FISH_VERSION");
+            std::env::set_var("PSModulePath", "/some/path");
+        }
+        let shell = detect_shell();
+        #[cfg(not(windows))]
+        assert!(matches!(shell, Shell::Posix));
+        #[cfg(windows)]
+        assert!(matches!(shell, Shell::PowerShell));
+        // Cleanup
+        unsafe {
+            std::env::remove_var("PSModulePath");
+        }
+    }
+
+    #[test]
+    #[serial]
+    fn test_detect_shell_fish() {
+        // SAFETY: This test runs in isolation with serial_test
+        unsafe {
+            std::env::set_var("FISH_VERSION", "3.7.0");
+            std::env::remove_var("PSModulePath");
+        }
+        let shell = detect_shell();
+        assert!(matches!(shell, Shell::Fish));
+        // Cleanup
+        unsafe {
+            std::env::remove_var("FISH_VERSION");
+        }
+    }
+
+    #[test]
+    #[serial]
+    fn test_detect_shell_posix_default() {
+        // SAFETY: This test runs in isolation with serial_test
+        unsafe {
+            std::env::remove_var("FISH_VERSION");
+            std::env::remove_var("PSModulePath");
+            std::env::remove_var("COMSPEC");
+        }
+        let shell = detect_shell();
+        #[cfg(not(windows))]
+        assert!(matches!(shell, Shell::Posix));
+    }
+
     #[test]
     fn test_format_export_posix() {
         let result = format_export(&Shell::Posix, "20.18.0");