test(routes): cover +error and forgot-password page branches

+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>

frontend/src/routes/error.svelte.test.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page as browserPage } from 'vitest/browser';
+
+const mockPage = {
+	status: 500,
+	error: { message: 'Internal Error' } as { message: string } | null
+};
+
+vi.mock('$app/state', () => ({
+	get page() {
+		return mockPage;
+	}
+}));
+
+afterEach(cleanup);
+
+async function loadComponent() {
+	return (await import('./+error.svelte')).default;
+}
+
+describe('+error.svelte', () => {
+	it('renders the page status code prominently', async () => {
+		mockPage.status = 404;
+		mockPage.error = { message: 'Not Found' };
+
+		const ErrorPage = await loadComponent();
+		render(ErrorPage);
+
+		await expect.element(browserPage.getByText('404')).toBeVisible();
+	});
+
+	it('renders the error message text from page.error.message', async () => {
+		mockPage.status = 500;
+		mockPage.error = { message: 'Database unavailable' };
+
+		const ErrorPage = await loadComponent();
+		render(ErrorPage);
+
+		await expect.element(browserPage.getByText('Database unavailable')).toBeVisible();
+	});
+
+	it('falls back to the literal "Internal Error" when page.error is null', async () => {
+		mockPage.status = 500;
+		mockPage.error = null;
+
+		const ErrorPage = await loadComponent();
+		render(ErrorPage);
+
+		await expect.element(browserPage.getByText('Internal Error')).toBeVisible();
+	});
+});