calendar: added support for custom calendar strings, and starting days

[xrtxn]

Closes #2517

Description

Added support for custom weekday, month, and starting days. I found this to be the best tradeoff in terms of performance and ergonomy.

Screenshot

Before	After

How to Test

First day

            DatePickerState::new(window, cx)
                .date_format("%Y. %m. %d.")
                .first_day_of_week(Weekday::Mon)

Week day and month day

        .weekday_names([
            t!("calendar.monday"),
            t!("calendar.tuesday"),
            t!("calendar.wednesday"),
            t!("calendar.thursday"),
            t!("calendar.friday"),
            t!("calendar.saturday"),
            t!("calendar.sunday"),
        ])
        .month_names([
            t!("calendar.january"),
            t!("calendar.february"),
            t!("calendar.march"),
            t!("calendar.april"),
            t!("calendar.may"),
            t!("calendar.june"),
            t!("calendar.july"),
            t!("calendar.august"),
            t!("calendar.september"),
            t!("calendar.october"),
            t!("calendar.november"),
            t!("calendar.december"),
        ])

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

Hi, weekday_names, month_names is not a good solution. I have make change of rust-i18n to support this feature and make PR in #2563

[xrtxn]

Thanks, this should work for me, this was my original idea too, but unsure how to implement it. However, please check out .first_day_of_week() as well, as this is still missing. If you rather I can implement the changes needed for this updated version,

[huacnlee]

first_day_of_week is ok to merge, you can cleanup you PR, then I will merge it.

[xrtxn]

Please check it out, I cleaned up the code

[huacnlee]

Thanks for this! The grid math for the start day is correct, and it stays backward-compatible for the default (Sunday).

One blocking issue: the weekday header row isn't reordered, so any first_day_of_week other than Sun makes the whole calendar misaligned.

In render_days (calendar.rs) the header is a fixed Sunday-first list and is rendered in that order regardless of the start day:

let weeks = [t!("Calendar.week.0") /*Sun*/, ..., t!("Calendar.week.6") /*Sat*/];
// weeks.iter().map(render_week)  // always Sun -> Sat

But days() now shifts the date columns to start on first_day_of_week. So with first_day = Mon, column 0 shows the "Sun" header on top of Monday's dates.

Example - Feb 2023 (the 1st is a Wednesday), first_day = Mon:

• date columns become Mon Tue Wed Thu Fri Sat Sun
• header still renders Sun Mon Tue ... -> off by one

Fix: rotate the weeks array by self.first_day_of_week.num_days_from_sunday() before rendering, using the same offset as the grid.

• test_days still only checks Weekday::Sun. Please add a Weekday::Mon case so the offset is locked in.