refactor: do not assign `this` to `self`

### Feature proposal

In fc-date-picker.js:76 (https://github.com/FlowingCode/YearMonthCalendarAddon/pull/104)

<img width="596" height="208" alt="Image" src="https://github.com/user-attachments/assets/34cc9441-20d8-4584-98dd-ce000a464d92" />

### Describe solution expectations

_No response_

### Additional information

_No response_