Improve type safety for useAuth hook with ensureSignedIn

[nicknisi]

This PR improves the TypeScript type safety of the useAuth hook by adding function overloads that guarantee user data is available when ensureSignedIn: true is used.

Changes

• Added function overloads for useAuth to provide stricter types
• When ensureSignedIn: true and loading: false, TypeScript now knows user is defined
• No runtime behavior changes, purely type-level improvements

Example Usage

// With ensureSignedIn: true
const auth = useAuth({ ensureSignedIn: true });
if (!auth.loading) {
  console.log(auth.user.email); // TypeScript knows user exists
}

// Without ensureSignedIn
const auth = useAuth();
if (!auth.loading && auth.user) {
  console.log(auth.user?.email); // Need to check for user
}

Testing

• No runtime tests needed as this is a type-level change
• Verified TypeScript compiler accepts the new types
• Verified existing usage patterns still work