feat: add backup progress estimation with time remaining

- Add estimate_backup_size() using du -sb before rsync starts
- Parse transfer speed from rsync output (KB/s, MB/s, GB/s)
- Calculate remaining time based on speed and remaining bytes
- Show estimated time remaining and size progress in UI
- Add format_time_remaining() for human-readable time display
- New UI elements: backup_time_label_ and backup_size_label_

src/main_window.h

@@ -90,6 +90,8 @@ private:
 	GtkWidget* backup_output_view_ = nullptr;
 	GtkWidget* backup_progress_ = nullptr;
 	GtkWidget* backup_status_label_ = nullptr;
+	GtkWidget* backup_time_label_ = nullptr;
+	GtkWidget* backup_size_label_ = nullptr;
 	GtkWidget* backup_cancel_btn_ = nullptr;
 	GtkWidget* backup_close_btn_ = nullptr;