[Task](ui): fix keyboard navigation in Select so first arrow key press focuses first menu item

[franzheidl]

Follow-up to #1748 / #1774.

When opening a Select via keyboard and navigating with arrow keys, several keystrokes are consumed before the first menu item appears focused. The menu items are there and selectable, but the focus ring does not appear until after a number of arrow presses.

Root Cause

The Select menu is rendered inside a portal using Floating UI. The floating container <div> receives getFloatingProps() from @floating-ui/react's useInteractions, which spreads interaction props — including a tabIndex — onto the div. This places the container and/or the ListboxOptions element into the browser's sequential focus order. Arrow key presses are consumed navigating through these invisible focusable elements before Headless UI's virtual list navigation kicks in for the items.

Fix

Add tabIndex={-1} explicitly to the floating container <div> in Select.component.tsx so it can still receive programmatic focus (required by Floating UI) but is excluded from sequential keyboard navigation.

Acceptance criteria

• Opening a Select via keyboard and pressing arrow down immediately highlights the first item
• Opening a Select via mouse and pressing arrow down immediately highlights the first item
• Existing keyboard open/close/select behaviour does not regress