fix(ui): install toast text not visible in light mode

[fgonzalezurriola]

• Changed background from --surface-float-base to --surface-raised-strong
• Replaced all --text-invert-* tokens with --text-* tokens
• Fixed typo: --text-invert-stronger doesn't exist in theme system
• Toast now adapts to light/dark theme like rest of UI

Before: Toast always dark in light mode, light in dark mode After: Toast follows theme - light bg in light mode, dark bg in dark mode

What does this PR do?

Fixes #7831

How did you verify your code works?

First I hardcoded in packages/app/src/pages/layout.tsx updateAvailable = true to force the toast to appear. Then I ran: cd packages/desktop bun run tauri dev