The provider tab strip is a horizontal-only Flickable. Touchpads flick it,
but a plain mouse only emits a vertical wheel, which Qt does not apply to
an axis the Flickable cannot flick, and no key was bound to it either. The
only way to reach a provider past the right edge was to drag the strip,
which nothing on screen suggests. contentX was never assigned anywhere, so
an auto-selected provider beyond the edge stayed invisible too.

Add three ways in:

- a WheelHandler limited to mouse devices maps the vertical wheel onto
  contentX, leaving the native touchpad flick untouched;
- Left/Right move focus between tabs, bounded to the strip via the focus
  chain, and focusing or selecting a tab scrolls it into view;
- the existing edge fades become buttons with a chevron, an accessible
  name and a pointing cursor, so the overflow is visible and clickable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>