feat: add fallback prop to AuthenticatedContent for unauthenticated state

[svidgen]

Summary

Adds an optional fallback parameter (third argument) to AuthenticatedContent. When the user is not authenticated, the component now renders the fallback node instead of an empty container.

Before

// Unauthenticated users see nothing — no way to show alternative UI
AuthenticatedContent(authApi, (user) => renderDashboard(user));

After

const loginPrompt = document.createElement('p');
loginPrompt.textContent = 'Please sign in to continue.';

AuthenticatedContent(authApi, (user) => renderDashboard(user), loginPrompt);
// Authenticated → dashboard
// Not authenticated → login prompt

Details

• Optional third fallback?: Node parameter — fully backward-compatible.
• No fallback provided → existing behavior (empty container when signed out).
• Updated JSDoc, README, and tests (5 test cases covering all states).
• Patch changeset for @aws-blocks/auth-common.

Testing

• npm run build passes for the package.
• All 53 unit tests pass (npm test -w packages/auth-common).

[changeset-bot]

🦋 Changeset detected

Latest commit: b8b2fe7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@aws-blocks/auth-common	Patch
@aws-blocks/blocks	Patch

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