Replace String::from_utf8().unwrap() with from_utf8_lossy()

Prevent panics on non-UTF-8 stderr output in invoke_script() (macOS)
and run_command() (Linux) by using String::from_utf8_lossy() which
replaces invalid sequences with the Unicode replacement character.

Closes #19

Author: Claude DevOps Engineer

src/linux.rs

@@ -59,7 +59,7 @@ fn run_command(command: &str, args: &[&str]) -> ShutdownResult {
             }
             Err(Error::new(
                 ErrorKind::Other,
-                String::from_utf8(output.stderr).unwrap(),
+                String::from_utf8_lossy(&output.stderr).into_owned(),
             ))
         }
         Err(error) => Err(error),

src/macos.rs

@@ -15,7 +15,7 @@ fn invoke_script(script: &str) -> ShutdownResult {
             }
             Err(Error::new(
                 ErrorKind::Other,
-                String::from_utf8(output.stderr).unwrap(),
+                String::from_utf8_lossy(&output.stderr).into_owned(),
             ))
         }
         Err(error) => Err(error),