style: Increase close button size in about dialog

- Increase padding and height of close button
- Make button fill full width with side padding
- Better visual balance in the dialog window

gui.py

@@ -378,11 +378,11 @@ class PDFtoICSGUI:
             bg="#3498db",
             fg="white",
             font=("Arial", 10, "bold"),
-            padx=30,
-            pady=8,
+            padx=50,
+            pady=12,
             cursor="hand2"
         )
-        close_btn.pack(pady=(0, 15))
+        close_btn.pack(pady=(10, 15), fill=tk.X, padx=20)
         
         
     def save_config(self):