fix: honor configured week start in calendar title

[thedhanawada]

What changed

The week header now passes the configured weekStartsOn to both date boundary helpers. With a Monday-start calendar on October 2, 2026, the header now shows September 28–October 4, matching the grid and getVisibleRange().

Why

The grid respected week-starts-on, but the header always used Sunday boundaries. This caused the displayed range to disagree with the dates in the calendar.

Test plan

• Existing tests pass (npm test -- --runInBand): 20 suites, 298 tests, including generated declarations and TypeScript consumer checks.
• Seven regression cases cover default/explicit Sunday starts, Monday starts, month/year boundaries, and changing the week start plus German locale after mount. The four Monday-related cases failed before the fix and pass afterward.
• Tests verify the rendered title, visible range, and first/last grid columns together.
• Production build, source Prettier check, and ESLint pass (0 errors; 12 existing warnings).
• Browser verification after the package is released and consumed by the website.

Breaking changes

None. No version bump or release is included.

[vercel]

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Project	Deployment	Actions	Updated
force-calendar-interface	Ready	Preview	Oct 2, 2026 1:44pm UTC