fix(ui): declare --destructive-foreground in :root and .dark

[rawdaymx]

Fixes #1240

What changed

--color-destructive-foreground is registered in @theme inline (so the
text-destructive-foreground utility exists), but --destructive-foreground itself was never
declared in :root or .dark — every other -foreground counterpart in the file has both.
Added the missing declarations:

:root {
  --destructive-foreground: oklch(0.985 0 0); /* white */
}
.dark {
  --destructive-foreground: oklch(0.145 0 0); /* near-black */
}

The two modes need different foregrounds — white only reads ~2.75:1 against --destructive in
dark mode, so this can't be fixed by hardcoding text-white at the two existing call sites
(prize-list-editor.tsx, sersavan/multi-select.tsx) instead of fixing the token.

Verification

Recomputed with standard WCAG 2.x contrast math:

	value
light: white vs --destructive	4.67:1
dark: near-black vs --destructive (dark)	6.89:1
control — white vs --destructive (dark), to confirm the two modes actually need different foregrounds	2.75:1 (fails)

Both real pairs clear the 4.5:1 WCAG 1.4.3 text floor.

pnpm --filter @chatbotx.io/ui check-types and pnpm --filter @chatbotx.io/ui test both pass
(13/13 test files, 56/56 tests) against this change.