Feat: Add weeknumbers to Calendar

[timunie]

What does the pull request do?

Adds week number display to both Calendar and CalendarDatePicker in month view (Fluent and Simple themes).

What is the current behavior?

Calendar and CalendarDatePicker only show day-of-week headers and date cells in month view. There is no way to display week numbers alongside the calendar grid.

What is the updated/expected behavior with this PR?

• A new week-number column can be shown on the left side of the month grid by setting IsWeekNumberVisible="True".
• The column header is configurable via WeekNumberHeader (e.g. "#", "CW", "KW", "Wk" — defaults to "#").
• The week numbering rule is configurable via WeekNumberRule using the new CalendarWeekNumberRule enum. Use WeekNumberRule="Iso" for ISO 8601 week numbering; other values (FirstDay, FirstFullWeek, FirstFourDayWeek) map to the BCL equivalents. Defaults to the current culture's rule.
• All properties are mirrored on CalendarDatePicker, where they propagate to the inner popup calendar.
• The calendar does not shrink when switching from month view to year/decade view. A new :hasweeknumbers pseudo-class on CalendarItem is used in the Fluent theme to hold the minimum width stable.

How was the solution implemented?

New type — CalendarWeekNumberLabel (Avalonia.Controls.Primitives)

A ContentControl subclass placed in column 0 of PART_MonthView. Exposes a :header pseudo-class for the top cell (row 0) so themes can style it differently from the data cells (rows 1–6).

New enum — CalendarWeekNumberRule (Avalonia.Controls)

Avalonia-specific enum replacing the BCL CalendarWeekRule on the WeekNumberRule property, with an additional Iso value:

public enum CalendarWeekNumberRule
{
    FirstDay = 0,         // maps to CalendarWeekRule.FirstDay
    FirstFullWeek = 1,    // maps to CalendarWeekRule.FirstFullWeek
    FirstFourDayWeek = 2, // maps to CalendarWeekRule.FirstFourDayWeek
    Iso = 3,              // explicit ISO 8601 shorthand
}

API diff

// Avalonia.Controls.Calendar — 3 new properties
+public static readonly StyledProperty<bool> IsWeekNumberVisibleProperty;
+public bool IsWeekNumberVisible { get; set; }                       // default: false

+public static readonly StyledProperty<CalendarWeekNumberRule> WeekNumberRuleProperty;
+public CalendarWeekNumberRule WeekNumberRule { get; set; }          // default: current culture

+public static readonly StyledProperty<object?> WeekNumberHeaderProperty;
+public object? WeekNumberHeader { get; set; }                       // default: "#"


// Avalonia.Controls.CalendarDatePicker — same 3 properties via AddOwner
+public static readonly StyledProperty<bool> IsWeekNumberVisibleProperty;
+public bool IsWeekNumberVisible { get; set; }

+public static readonly StyledProperty<CalendarWeekNumberRule> WeekNumberRuleProperty;
+public CalendarWeekNumberRule WeekNumberRule { get; set; }

+public static readonly StyledProperty<object?> WeekNumberHeaderProperty;
+public object? WeekNumberHeader { get; set; }


// Avalonia.Controls.Primitives.CalendarItem — new pseudo-class
-[PseudoClasses(":calendardisabled")]
+[PseudoClasses(":calendardisabled", ":hasweeknumbers")]


// Avalonia.Controls.Primitives.CalendarWeekNumberLabel — new public type
+public sealed class CalendarWeekNumberLabel : ContentControl
+{
+    public bool IsHeader { get; internal set; }  // sets :header pseudo-class
+}

// Avalonia.Controls/Calendar/DateTimeHelper.cs
+public static int GetWeekOfYear(DateTime date, CalendarWeekNumberRule rule, DayOfWeek firstDayOfWeek)

ISO 8601 week number fix — DateTimeHelper.GetWeekOfYear

.NET's Calendar.GetWeekOfYear with FirstFourDayWeek + Monday incorrectly returns week 53 for late-December dates that ISO 8601 assigns to week 1 of the next year (e.g. 2018-12-31 → ISO week 1 of 2019). Both the Iso shorthand and the explicit FirstFourDayWeek + Monday combination delegate to ISOWeek.GetWeekOfYear to avoid this bug:

public static int GetWeekOfYear(DateTime date, CalendarWeekNumberRule rule, DayOfWeek firstDayOfWeek)
{
    var mappedRule = (System.Globalization.CalendarWeekRule)(int)rule;

    if (rule == CalendarWeekNumberRule.Iso ||
        (mappedRule == CalendarWeekRule.FirstFourDayWeek && firstDayOfWeek == DayOfWeek.Monday))
        return ISOWeek.GetWeekOfYear(date);

    return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, mappedRule, firstDayOfWeek);
}

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #7976

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065222-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065248-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[robloo]

As mentioned above, if the CalendarWeekNumberLabel goes away it would be needed to add CalendarViewWeekNumberForeground and related resources here.

[robloo]

IMO an entire new primitive probably isn't needed for this concept. It can be done with text and light-weight styling resources alone.

That in turn allows room for future expansion in this area. For example -- we might want to make it interactive as a button in some way, the calendar might get a week ONLY view, etc.

I'm also not a fan of the legacy term "Label" being used for new controls. That just isn't done in modern XAML.

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065706-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]