feat(ui): QueryaSplitButton, QueryaMenuButton and QueryaDialogActions #1335

Part of #1331. Needs QueryaButton.

Problem

The compositions that went wrong in the review are two buttons placed side by side: Execute + its menu (generic_sql_workspace.dart), Export / Views menus next to action buttons (ERD), and every dialog's hand-assembled footer.

Scope

• QueryaSplitButton: a primary action and a chevron that opens a menu, one border and radius, equal height, shared hover / focus / loading, one tooltip each. Replaces Execute + QueryaActionMenu.
• QueryaMenuButton: a button that opens a menu (label or icon, chevron, isActive while open); replaces the trigger of QueryaActionMenu / ExportMenuButton, the menu surface stays QueryaDropdown's.
• QueryaDialogActions: the footer row of the standard: order, spacing 8, lg size, right alignment, a divider; takes cancel, secondary, primary / destructive slots and enforces one primary; handles Esc -> cancel and Enter -> primary through the dialog's focus scope.
• QueryaButtonGroup for segmented actions that are not tabs.

Acceptance

• The Execute split button, the ERD toolbar and the results toolbar are built from these with no SizedBox between halves.
• A dialog built with QueryaDialogActions cannot show two primaries (assert in debug).
• Menu buttons, split buttons and footers have widget tests, including keyboard navigation.