feat(ui): add DatePicker component (#34184)

* feat(ui): add DatePicker component

Add a single-date picker built on React Aria's DatePicker, mirroring the
existing DateRangePicker implementation. Includes the date field with
segmented input, calendar popover, BUI design tokens, bg consumer
pattern, and full keyboard/screen reader accessibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Erik Hughes <erikh@spotify.com>

* fix(ui): address DatePicker PR feedback

- Remove unused dataAttributes spread from DatePickerGroup
- Mark DatePickerGroupDefinition and DatePickerCalendarDefinition as
  public so CSS class name changes appear in API reports
- Add Affected components line to changeset

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Erik Hughes <erikh@spotify.com>

* fix(ui): restore dataAttributes spread in DatePickerGroup

The bg: 'consumer' config on DatePickerGroupDefinition emits
data-on-bg attributes via useDefinition. Without spreading
dataAttributes onto <Group>, the CSS [data-on-bg] selectors
never match and background auto-increment doesn't work.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Erik Hughes <erikh@spotify.com>

---------

Signed-off-by: Erik Hughes <erikh@spotify.com>
Co-authored-by: Erik Hughes <erikh@spotify.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.changeset/ui-date-picker.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Added new `DatePicker` component — combines a date field and a calendar popover for selecting a date, built on React Aria with full keyboard and screen reader accessibility. Uses BUI design tokens throughout, including auto-incremented backgrounds via the bg consumer pattern.
+
+**Affected components:** DatePicker