Certificate Email Sender GUI v1.4

✅ GUI Added
An intuitive and simple Tkinter-based GUI replaces command-line interaction for ease of use.

📬 Summary Email Feature
After completing the mailing process, a summary report is automatically sent to the configured sender email. It includes total sent, success, and failed counts.

📎 Optional Certificate Attachment
You can now choose whether or not to attach PDF certificates in the emails.

🧾 Real-Time Log & Progress Bar
Track email sending progress and errors with an interactive progress bar and real-time log output.