dev(ui): UI kit gallery: every control, variant, size and state in every theme #1336

Part of #1331.

Why

Standards drift when nobody can see them side by side. A gallery makes the review of this epic (and of every later control) a look at one screen instead of a hunt through features.

Scope

• A debug / profile-only page (not in release builds), reachable from the command palette: UI kit gallery.
• Sections: buttons (variant x size x state x with / without icon, icon-only, loading, long label, narrow), split and menu buttons, dialog footers, toolbars at sm / md / lg with a dropdown, search field and tab strip in the same row, tooltips, focus ring.
• A theme switcher (built-in themes, light / dark, an imported VS Code theme) and text-scale / uiScale sliders on the page.
• The page is the source of the layout tests of the epic's test issue.

Acceptance

• The gallery opens without a connection and lists every kit control.
• A new control that is added to widgets.dart without a gallery entry fails a test.