Proper Radix tooltip for the queue's "why flagged" hint

The hand-rolled hover bubble was a full-width black bar that covered the row
and just echoed the terse scenario label. Replace it with a real Radix tooltip
(components/ui/tooltip.tsx, styled in the app's ink register) so positioning,
collision-avoidance, the arrow, the portal, and a11y are handled, no clipping.

And make the copy actually useful: scenarioExplain() turns the label into the
reason an AP reviewer wants, e.g. "A bill with this number is already posted in
the ERP" instead of "Duplicate invoice".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: DylanMerigaud