+error.svelte: vi.mock('$app/state') drives the page state so each test
can assert one of the three rendering branches — populated error message,
distinct status code, and the 'Internal Error' fallback when page.error
is null.
forgot-password/+page.svelte: prop-driven tests for the four states —
default form, success banner, error message inside the form, and the
back-to-login link href.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>