fix: enhance date formatting for calendar awareness

[devalimotahari]

Summary

This pull request fixes an issue where non-Gregorian calendars (e.g., Jalali/Persian) do not display correct day or month values when a localized language code is passed (e.g., fa-IR-u-ca-persian).

Currently, frappe-gantt relies on raw Date object values from this.get_date_values(date), which always use the Gregorian calendar. As a result, when users select languages that use calendar systems like Jalali, the day and month fields are rendered incorrectly.

Problem Example

When using:
gantt = new Gantt('#gantt', tasks, { language: 'fa-IR-u-ca-persian' });

Expected: Days and months should follow the Jalali calendar.
Actual: Days of months are displayed incorrectly (see screenshot).

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Fix Implemented

• Switched to Intl.DateTimeFormat for extracting year, month, and day values.

• This ensures that values respect the active calendar system defined in the lang parameter.

• The new format_map separates calendar-aware values (YYYY, MM, DD, D, MMMM, MMM) from time values (HH, mm, ss, SSS), ensuring correct rendering across different locales and calendars.

Example:
YYYY: yearFmt.format(date), MM: month2Fmt.format(date), DD: day2Fmt.format(date), D: dayFmt.format(date), MMMM: month_name_capitalized, MMM: monthShortFmt.format(date)

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Requirements / Notes

• ✅ Tested with fa-IR-u-ca-persian (Jalali calendar).

• ✅ Backward compatible with default en (Gregorian calendar).

• ✅ No external dependencies added — relies only on Intl.DateTimeFormat.

• 📖 Reference: Unicode Locale Identifiers (e.g., fa-IR-u-ca-persian).

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Impact

This change improves internationalization support in frappe-gantt and ensures correct date rendering for users working with non-Gregorian calendars.