fix(a11y): add combobox semantics and selected item announcement to QueryaDropdown #1371

Summary

In Querya UI Kit QueryaDropdown (lib/shared/widgets/querya_dropdown.dart, lines 207–293, 300–350):

QueryaDropdown handles keyboard events (Enter, Space, ArrowDown) and focus styling, but contains zero Semantics widgets.

To assistive technologies (screen readers like NVDA, Orca, VoiceOver):

• The trigger is announced as plain unlabelled text/icons, with no indication that it is an interactive dropdown or combobox.
• Screen readers cannot detect whether the menu is currently expanded or collapsed.
• Menu items inside the popup do not expose Semantics(selected: isSelected, button: true).

Scope

• In QueryaDropdown._buildTrigger, wrap the trigger in material.Semantics:
  • button: true
  • expanded: controller.isOpen
  • value: label
  • hint: widget.hint
• In _QueryaDropdownMenuItem, wrap items in material.Semantics(button: true, selected: item.value == widget.value).
• Note: This component is currently part of Epic epic(ui): migrate QueryaDropdown, QueryaModalDialog, and QueryaSearchField off shadcn_flutter to Flutter primitives #1365 (migrating off shadcn to native Flutter primitives) and should incorporate these semantics into the new implementation.