fix: improve toast contrast and stacking

[saurabhhhcodes]

Summary

• use a dark, high-contrast toast surface for readable notifications
• align the published usage snippets with the implementation
• raise the toast container stacking context and keep auto-dismiss timing synchronized
• add an accessible label to the dismiss action

Validation

• git diff --check
• npm run build (passes with pre-existing lint warnings)
• CI=true npm test -- --watchAll=false --runInBand (3 tests passed)

Closes #506

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