fix(ui): dropdown menu is narrower than its trigger #1444

Problem

The menu of QueryaDropdown is as wide as its longest item, not as its trigger (Theme mode in Preferences: trigger about 360 px, menu about 185 px), so the open menu looks detached from the field.

Scope

• QueryaDropdown menu: minimum width equal to the trigger's width, max width from the viewport.
• Widget test for the menu width against the trigger.

Found while driving a Linux debug build of dev after the button migration (UI consistency review). Part of epic #1331.