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

[ZhuchkaTriplesix]

Closes #1334 (part of epic #1331)

Replaces #1406, which was merged into the #1404 branch after #1404 had already gone to dev, so the button never reached dev and never ran CI (the workflow only runs for PRs into main / dev). Same commit, now targeting dev.

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)

• Parameters: size (default from QueryaControlScope, then md), icon, trailing, shortcutHint, loading, tooltip, isActive, compact, autofocus, semanticLabel, tone.
• States: hover / pressed tints, keyboard-only focus ring (foreground decoration, no layout shift), disabled, loading (width kept, not pressable), selected.
• Enter / Space activate; semantics button, label, enabled, selected.
• One-line ellipsizing label, square icon-only, sizes from QueryaControlSize, colours from the workbench, motion from QueryaMotion.

Not in this PR

QueryaActionButton is untouched (own tests, 17 call sites; swapped during the migration), QueryaSplitButton / QueryaMenuButton / QueryaDialogActions are #1335.

Tests

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

Not run locally; CI is the check.