A Calendar Widget for Extended Date Inputs

### What is the problem or limitation you are having?

Right now all date inputs are field-like; add Calendar to display a full calendar for picking dates.

### Describe the solution you'd like

A new interface for abstracting calendar views. Wrap these native APIs:

macOS: Style set to https://developer.apple.com/documentation/appkit/nsdatepicker/style/clockandcalendar?language=objc

iOS: https://developer.apple.com/documentation/uikit/uicalendarview?language=objc

GTK: Gtk.Calendar

Winforms: https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.monthcalendar?view=windowsdesktop-9.0

Android: https://developer.android.com/reference/android/widget/CalendarView

After that, replace the current large GTK date picker implementation with a TextInput or maybe two or three or other stuff.

### Describe alternatives you've considered

Status quo, but we don't have large inline calendars to pick from then

### Additional context

_No response_

The issue names the existing large GTK date picker implementation and native calendar APIs for macOS, iOS, GTK, WinForms, and Android, but no repository paths or tests. Begin by locating the GTK date-picker code and platform backend interfaces; completion would require an agreed cross-platform calendar abstraction and corresponding backend coverage.

Written by the indexing model from the issue text.