Fix bad contrast for highlighted search results in tables (#9863)

* Fix bad contrast for highlighted search results in tables

`searchMatchColorOpaque` and `activeSearchMatchColorOpaque` are blended
over the row background to highlight search matches. They were migrated
from `withOpacity(0.5)` to `withValues(alpha: 255 / 2)` in #8784; since
`withValues` expects alpha in the range 0.0-1.0, `255 / 2` clamps to fully
opaque. Matched rows then got a solid yellow/orange background while the
row text kept its theme color, making it unreadable (especially in dark
mode).

Restore the intended 0.5 alpha so the highlight stays translucent and the
row text remains legible, and add a regression test.

Fixes #9010

* Rename search match colors to *Translucent; add release note