feat(core): Calendar weekStartsOn accepts three-letter day names

[durvesh1992]

Summary

Calendar's weekStartsOn only accepted a numeric 0–6, which isn't self-documenting at the call site (#2843). It now also accepts a three-letter day name ('sun'–'sat', case-insensitive):

<Calendar weekStartsOn="mon" />   // same as weekStartsOn={1}

• Numbers keep working unchanged — the union is a pure superset, so it's non-breaking.
• The value is normalized to a numeric DayOfWeek at the component boundary, so all downstream date math is untouched.
• Adds an exported DayOfWeekName type and a small pure normalizeDayOfWeek helper in utils/dateTypes (also re-exported from Calendar).
• Unrecognized strings fall back to 0 (Sunday).

Test plan

• normalizeDayOfWeek unit tests: numeric passthrough, all 7 names → index, case-insensitivity, unknown → 0.
• Calendar render tests: weekStartsOn="mon" and "WED" produce the same first column header as the numeric equivalents. 36/36 tests pass across both files.
• tsc --noEmit clean (0 errors); updated prop JSDoc + Calendar.doc.mjs (EN/ZH/dense).

Closes #2843

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[cixzhang]

Nice, this is a good improvement for making the option more readable when written