Replace the Playwright spec from #104065 with one jsdom invariant in the
existing GroupMentionInput suite: the popover carries `bg-(--ui-bg-elevated)`
and never a `--ui-bg-{primary,secondary,tertiary}` fill token. The Desktop
E2E job is `if: false` in ci.yaml, so the 96-line spec (two bots, a group,
a 32-paragraph post, a theme toggle) would never run; the token → opaque
surface chain was verified once in headless Chromium (alpha 62 → 255).
e6fa1fd474