Added unit tests for pure logic. Refs #2

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/error.rs

@@ -101,4 +101,50 @@ impl From<std::io::Error> for AppError {
         log_error(&value);
         AppError::FileIOError(value.to_string())
     }
-}
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chrono::NaiveTime;
+
+    #[test]
+    fn error_messages() {
+        assert_eq!(AppError::HourParseError.to_string(), "Hour parse error");
+        assert_eq!(AppError::SlotParseError.to_string(), "Book slot parse error");
+        assert_eq!(AppError::ServerError("boom".to_string()).to_string(), "Server error: boom");
+        assert_eq!(AppError::FatalError("boom".to_string()).to_string(), "Fatal error: boom");
+        assert_eq!(AppError::MailAddrParseErr("boom".to_string()).to_string(), "Cannot parse email address: boom");
+        assert_eq!(AppError::MailSendError("boom".to_string()).to_string(), "Cannot send email: boom");
+        assert_eq!(AppError::FileIOError("boom".to_string()).to_string(), "IO error: boom");
+    }
+
+    #[test]
+    fn time_parse_error_becomes_hour_parse_error() {
+        let err = NaiveTime::parse_from_str("nonsense", "%H:%M").unwrap_err();
+
+        assert_eq!(AppError::from(err), AppError::HourParseError);
+    }
+
+    #[test]
+    fn server_fn_error_keeps_message() {
+        let err = AppError::from(ServerFnError::new("boom"));
+
+        assert!(matches!(err, AppError::ServerError(ref m) if m.contains("boom")));
+    }
+
+    #[cfg(feature = "ssr")]
+    #[test]
+    fn io_error_becomes_file_io_error() {
+        let err = std::io::Error::new(std::io::ErrorKind::NotFound, "missing");
+
+        assert!(matches!(AppError::from(err), AppError::FileIOError(ref m) if m.contains("missing")));
+    }
+
+    #[test]
+    fn app_error_converts_to_server_fn_error() {
+        let err: ServerFnError = AppError::HourParseError.into();
+
+        assert!(err.to_string().contains("Hour parse error"));
+    }
+}