[WC-3492]: fix(calendar-web): apply custom Header day format to column headers

[rahmanunver]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

What: In the Calendar widget's Custom view, the "Header day format" property now applies to the day, week, and month column headers — not just the toolbar title.

Why: A customer (Mendix 10.24.19, Calendar module 2.4.0) set Header day format: EE dd-MM but the day-column header still rendered the default 07 Tue.

react-big-calendar uses two distinct sets of format keys:

• dayHeaderFormat / monthHeaderFormat — the toolbar navigation title
• dayFormat (week/day columns) and weekdayFormat (month weekday headers) — the per-column headers, the "07 Tue" text the user actually sees

CalendarPropsBuilder.buildFormats wired customViewHeaderDayFormat only into the toolbar-title keys, so the column headers kept RBC's date-fns default dayFormat: 'dd eee' and the user's pattern was silently dropped.

Fix: buildFormats now also sets formats.dayFormat (day/week columns) and formats.weekdayFormat (month) from the same customViewHeaderDayFormat pattern. RBC exposes a single shared dayFormat for day and week, so when both items set different patterns the week pattern wins (consistent with the existing timeGutterFormat precedence) and a console.warn is logged on collision. The custom work_week view spreads formats into RBC's TimeGrid, so it inherits the new dayFormat automatically.

Added unit tests for both formats, the collision precedence + warning, the no-format regression, and the existing time-gutter wiring (21/21 pass).

What should be covered while testing?

1. Calendar in Custom view with day and week toolbar items.
2. Set Header day format = EE dd-MM and Time gutter format = HH:mm.
3. Day/week column headers should render e.g. Tue 07-07 (not 07 Tue); time gutter shows 24h 14:00.
4. Switch to month view — weekday headers reflect the pattern.
5. Regression: with no custom format set, headers fall back to RBC defaults unchanged.