fix(ui): use explicit light text colors for toast on dark float surface

[zerone0x]

Summary

Fixes #7833

Toast text is unreadable in Light mode because dark text appears on a dark background.

Root Cause

The toast component uses --surface-float-base which is a dark surface in both light and dark modes. However, it was using --text-invert-* CSS variables for text colors. These variables don't properly resolve to light colors in light mode due to how neutralAlpha is calculated in the theme resolver (resolve.ts).

This causes:

• Light mode: Dark text on dark toast background (unreadable)
• Dark mode: Works correctly

Changes

Replace --text-invert-* variables with explicit --smoke-dark-* colors which are always light colors suitable for dark backgrounds:

Element	Old Variable	New Variable	Color
Title	--text-invert-strong	--smoke-dark-12	#f1ecec (near-white)
Description	--text-invert-base	--smoke-dark-11	#b7b1b1 (light gray)
Icon	--text-invert-stronger	--smoke-dark-12	#f1ecec
Action (weak)	--text-invert-weak	--smoke-dark-9	#716c6b (medium gray)
Action (first)	--text-invert-strong	--smoke-dark-12	#f1ecec

Test plan

• [x] UI package typechecks pass
• [ ] Visual verification: Toast text is readable in both Light and Dark modes

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