fix: GECOS subfield trimming and trailing backslash handling (v0.3.1)

display_name() now returns only the first GECOS subfield (before comma)
instead of the full GECOS string with room numbers and phone extensions.

split_shell_words() returns None for trailing backslashes instead of
silently ignoring them.

src/greeter.rs

@@ -47,14 +47,16 @@ fn split_shell_words(s: &str) -> Option<Vec<String>> {
             }
             '\\' if in_double => {
                 // In double quotes, backslash escapes the next char
-                if let Some(next) = chars.next() {
-                    current.push(next);
+                match chars.next() {
+                    Some(next) => current.push(next),
+                    None => return None, // Trailing backslash
                 }
             }
             '\\' if !in_single && !in_double => {
                 // Outside quotes, backslash escapes the next char
-                if let Some(next) = chars.next() {
-                    current.push(next);
+                match chars.next() {
+                    Some(next) => current.push(next),
+                    None => return None, // Trailing backslash
                 }
             }
             c if c.is_whitespace() && !in_single && !in_double => {
@@ -1189,6 +1191,12 @@ mod tests {
         );
     }
 
+    #[test]
+    fn shell_words_trailing_backslash() {
+        assert_eq!(split_shell_words(r"foo\"), None);
+        assert_eq!(split_shell_words(r#""foo\"#), None);
+    }
+
     // -- login_worker tests --
     // These use a real Unix socket pair via UnixListener to simulate greetd.