Add a date picker (calendar) widget

[AlexSkrypnyk]

Summary

Add a date-picker widget that renders a navigable month calendar.

Details

There is no date type in the current widget set, so dates are entered as free text, which is error-prone.

Add a date-picker widget that shows a month calendar grid: arrow (and optional vim) keys move by day, page keys change month. Support a configurable minimum and maximum date, a configurable week-start day, and a default date. It returns a normalized date value.

Acceptance criteria

• A new widget renders a month calendar and returns a selected date in a documented, normalized format.
• Navigation moves by day and by month; min/max bounds are enforced; the week-start day is configurable.
• The widget honours required/validate/default and participates in the fluent builder.
• Headless (JSON/env) collection accepts a date string and the JSON schema describes the field and its format.
• Rendering is theme-driven and works in Unicode and ASCII display modes.
• Unit tests cover navigation, bounds, week-start and headless parity.
• The widget is documented in the README widgets section.