fix(timezone): preserve timezone context when adding calendar units

[SHARMA1525]

Fix issue where adding days/weeks/months/years to timezone-aware dayjs objects did not preserve timezone context correctly, causing incorrect valueOf() results when converting back to timezone.

The fix overrides the add() method for timezone-aware objects to:

1. Format the date in timezone context
2. Add calendar units without timezone conversion
3. Convert back to timezone with keepLocalTime=true

This ensures all three approaches produce identical results:

• dayjs.tz(time, tz).add(1, 'day')
• dayjs.tz(dayjs.tz(time, tz).add(1, 'day').valueOf(), tz)
• dayjs.tz(dayjs.tz(time, tz).add(1, 'day').startOf('millisecond').valueOf(), tz)

Fixes issue where valueOf() and format() results were inconsistent after adding calendar units to timezone-aware objects.

Linked Issue
#2957

[Xiphoseer]

add is not the only function where this goes wrong and it's not the only DST bug as you can see from the issue tracker. set is another candidate. What does sort of work is to patch the private init function to recompute the offset. I've built a custom dst plugin that does that but not sure I can share it.

[SHARMA1525]

I’ve raised a new PR that implements the suggested fix by patching the private init() function to recompute the timezone offset.

This ensures DST offsets remain consistent across all methods (.add(), .set(), .startOf(), etc.), not just .add().

All test suites (93/93) pass successfully.

Please take a look when you get a chance thank you again for your guidance!