feat(ui): QueryaMenuButton, QueryaSplitButton and QueryaDialogActions

[ZhuchkaTriplesix]

Part of #1335 / epic #1331. Builds on QueryaButton (#1408, in dev) and the control scale (#1404).

What

The compositions the review found hand-assembled, all without a shadcn_flutter import:

• QueryaMenuButton<T>: a QueryaButton with a trailing chevron that opens a menu; isActive while open; the same popup surface as QueryaDropdown (shared in querya_menu_surface.dart). Replaces the trigger of QueryaActionMenu / ExportMenuButton.
• QueryaSplitButton<T>: a main action and a chevron half as one control: one clip and radius, equal height, a 1 px divider instead of a gap, a tooltip per half, loading disables both. A secondary variant draws one outline around both halves.
• QueryaDialogActions: the footer of the standard: cancel (ghost) first, secondary actions, then one main action (primary or destructive), lg, 8 apart, right-aligned under a divider. Debug asserts: not both main actions, and each slot has its variant. Esc runs cancel inside the footer; QueryaDialogActions.bindings(...) gives the Esc / Enter map for a dialog-level CallbackShortcuts.

QueryaButton gets what they need: a public general constructor (variant: as a parameter), a borderRadius override (the halves of a split button square their inner corners), and an IconTheme so a trailing icon takes the button's colour and size.

Not in this PR

• QueryaButtonGroup (segmented non-tab actions) is left out: nothing in the app needs it until the toolbars are migrated.
• Replacing Execute + menu, the ERD toolbar and the dialog footers with these is the migration (refactor(ui): migrate toolbars, panels and the self-made buttons to QueryaButton #1337 / refactor(ui): migrate dialogs and forms to QueryaButton and QueryaDialogActions #1338); nothing is rewired here, so there is no behaviour change in the app.
• QueryaActionMenu itself is untouched (it still uses shadcn's OutlineButton).

Tests

• querya_menu_and_split_button_test.dart: open and pick, same height as a button, disabled does not open, Enter on the focused trigger opens; split: main runs, chevron opens and picks (main not run), both halves have the control height, loading disables both, only a 1 px divider between halves.
• querya_dialog_actions_test.dart: order, lg height and 8 px spacing, right alignment, Esc runs cancel, bindings for Esc / Enter and not for a disabled or loading main action, both-main-actions assertion, wrong-variant cancel rejected in debug.

Not run locally; CI is the check.