Support Hexadecimal Page Numbers (e.g. 1FF, 12E)

- Refactored  to parse page numbers as nibbles ((T<<4)|U) instead of decimal, preventing collisions between hex and decimal pages.
- Updated  to format page IDs as Hex.
- Updated  to display page IDs as Hex in the list.
- Ensures filler/housekeeping pages are correctly isolated.

src/teletext/ui.py

@@ -496,7 +496,8 @@ class MainWindow(QMainWindow):
         sorted_keys = sorted(self.page_groups.keys())
         
         for mag, pnum in sorted_keys:
-            label = f"{mag}{pnum:02d}"
+            # Display as Hex
+            label = f"{mag}{pnum:02X}"
             item = QListWidgetItem(label)
             item.setData(Qt.ItemDataRole.UserRole, (mag, pnum))
             self.page_list.addItem(item)