[Bug]: weekStartsOn does not work correctly with non-English locale

[ploca14]

Environment

Development OS: Windows 10 19043.1110
Node version: 16.0.0
Package manager: pnpm@8.6.0
Reka UI version: 2.0.0
Vue version: 3.0.0
Locale tested: cs-CZ
Nuxt mode: universal

Link to minimal reproduction

https://stackblitz.com/edit/c5xh3s73?file=src%2FApp.vue

Steps to reproduce

1. Render RangeCalendarRoot or CalendarRoot and pass weekStartsOn=1 and locale='cs-CZ'.
2. Observe that the calendar grid starts on Tuesday, not Monday.
3. Repeat with weekStartsOn=1 and locale='en-US': calendar starts on Monday as expected.
4. This shows weekStartsOn logic does not work correctly for Czech or other non-English locales.

Describe the bug

When weekStartsOn is set to 1 (Monday) and locale is set to cs-CZ, the calendar week starts on Tuesday instead of Monday. This does not happen when locale is en-US.

It appears that week grid calculation internally uses getDayOfWeek(date, locale), which maps weekdays differently based on locale. The week arithmetic should be locale-insensitive: weekStartsOn=1 should always mean Monday, regardless of locale.

Expected behavior

Passing weekStartsOn=1 should always make the week start on Monday, independent of locale. Locale should only affect weekday names, not grid calculation.

Context & Screenshots (if applicable)

No response

[kricsleo]

Duplicate of #2157.
People in different countries (locales) consider different days to be the "first day of the week," not always a specific day.