feat(ui): QueryaButton on Flutter primitives, no shadcn

[ZhuchkaTriplesix]

Closes #1334 (part of epic #1331)

Stacked on #1404 (control metrics, issue/1333-control-metrics-tokens): this PR targets that branch so the diff shows only the button. Once #1404 is merged, retarget to dev (or merge #1404 first and GitHub will do it).

What

lib/shared/widgets/querya_button.dart, exported from widgets.dart, with no shadcn_flutter import:

QueryaButton.primary(label: 'Save', onPressed: _save)
QueryaButton.secondary(label: 'Explain', icon: Icons.account_tree_outlined, onPressed: _explain)
QueryaButton.ghost(label: 'Cancel', onPressed: _close)
QueryaButton.destructive(label: 'Delete', onPressed: _delete)
QueryaButton.secondary(label: 'Rollback', tone: QueryaButtonTone.destructive, ...)  // red text
QueryaButton.icon(icon: Icons.refresh, tooltip: 'Refresh', onPressed: _reload)

Common parameters: size (default from QueryaControlScope, then md), icon, trailing, shortcutHint, loading, tooltip, isActive, compact, autofocus, semanticLabel.

• States: hover / pressed tints, keyboard-only focus ring (FocusableActionDetector.onShowFocusHighlight, drawn as a foreground decoration so it never shifts layout), disabled (50 %), loading (spinner in the icon's place, or over the hidden label, width kept; not pressable), selected (isActive).
• Colours from context.workbench and the theme foreground; transitions use QueryaMotion (instant under reduced motion).
• Keyboard: Enter and Space activate; semantics: button, label, enabled, selected, hint while loading.
• Layout: the label is one line and ellipsizes; icon-only is square; height / padding / font / icon from QueryaControlSize.

Not in this PR

• QueryaActionButton is untouched. The issue asks for a deprecated thin wrapper, but it has its own tests that look for shadcn button types and 17 call sites; it is swapped during the migration (refactor(ui): migrate toolbars, panels and the self-made buttons to QueryaButton #1337) so each change stays reviewable.
• QueryaSplitButton, QueryaMenuButton, QueryaDialogActions are feat(ui): QueryaSplitButton, QueryaMenuButton and QueryaDialogActions #1335.
• Gallery (dev(ui): UI kit gallery: every control, variant, size and state in every theme #1336) and the theme-switch / row-height tests (test(ui): widget and layout tests for the button family and for one height per row #1340).

Tests

test/shared/widgets/querya_button_test.dart: tap, disabled and loading do not fire, Enter / Space activate, height per size, icon-only is square, compact keeps the label as tooltip, loading keeps width, long label ellipsizes without overflow, tooltip delay is kQueryaTooltipWait, semantics flags, and a source guard that the file imports no shadcn_flutter.

Not run locally; CI is the check.