docs(toast): add usage guide covering duration, density, quiet mode, and actions

[jabir-dev788]

Summary

Adds docs/components/toast.md, documenting the public API and behavioral
contracts of the toast system (useToast / ToastProvider) that previously
only existed as inline comments in toast-provider.tsx.

Covers

• showSuccess / showError / dismissToast signatures and return values
• The 'suppressed' return under quietMode
• role="status" vs role="alert" and polite/assertive live-region semantics
• Density-driven stacking gap
• MAX_VISIBLE_TOASTS cap and eviction order
• The action: { label, onClick } contract, including dismiss-on-fire behavior
• Copy-pasteable PreferencesProvider + ToastProvider mounting example
• Return-value reference table

Verification

Every documented behavior was cross-checked against toast-provider.tsx
source and toast-provider.test.tsx assertions to avoid drift. No source
files were modified — documentation only.

Scope note

The issue's general guidelines mention 95% test coverage; this is a
documentation-only PR with no code changes, so no new tests were added,
per the issue's own "Suggested execution" section (snippets verified by
mirroring existing test patterns instead).

Checks

• npm run lint
• npm test
• npm run build