feat: expose isAnimated flag to renderDay for floating day header (#2721)

[kesha-antonov]

Closes #2721

Problem

renderDay is used for both the inline day separators and the floating/animated day header that sticks to the top while scrolling. There was no way to distinguish the two inside a custom renderDay, so users were forced to give both the same look.

Fix

Pass an isAnimated flag in the DayProps handed to renderDay:

• true for the floating animated header
• false (or undefined) for the inline separators

A single renderDay can now branch on it:

renderDay={(props) =>
  props.isAnimated
    ? <FloatingDayHeader {...props} />
    : <Day {...props} />
}

dateFormat / dateFormatCalendar already flow through to the floating header, so custom date formats stay in sync there too (the second half of the issue).

This is non-breaking - isAnimated is an additive optional field on DayProps; existing renderDay callbacks keep working unchanged.

Validation

Verified on the Android emulator with a branching renderDay: the inline separators render one style while the floating header renders a distinct one as it appears during scroll.