chore(ui): remove the remaining shadcn buttons and add guard tests for the button standard #1339

Part of #1331. Last, after the two migration issues.

Scope

• Delete the deprecated QueryaActionButton wrapper and the QueryaIconButtonDensity aliases.
• Guards in test/guards/ (same style as no_raw_material_dialogs_test.dart):
  • no_shadcn_buttons_test.dart: PrimaryButton, OutlineButton, GhostButton, DestructiveButton, SecondaryButton, LinkButton, ButtonSize, ButtonDensity and ButtonStyle.* of shadcn_flutter are forbidden outside an explicit allow-list (for third-party code that is replaced later).
  • no_raw_material_buttons_test.dart: IconButton, TextButton, ElevatedButton, OutlinedButton, FilledButton forbidden; extends the existing results-only guard to the whole app.
  • no_ad_hoc_button_classes_test.dart: a class named *Button outside lib/shared/widgets/, and a button-like InkWell / GestureDetector (a Text or Icon child with onTap) outside the kit, fail.
  • button_literals_test.dart: no waitDuration: Duration(...) literal, no BorderRadius.circular(n) on a kit control.
• Update docs/ui-kit.md's guard table and the Obsidian note.

Acceptance

• The guards pass on dev; the allow-list is empty or each entry names a follow-up issue.
• grep -r "shadcn_flutter" lib | grep -i button is empty.