feat(auth): build login page with LoginForm, brand panel, and title

Replaces placeholder with full login page: brand panel left,
LoginForm right, svelte:head title, signup link, no-nav-chrome.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/routes/(public)/login/page.test.ts

@@ -0,0 +1,44 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen } from '@testing-library/svelte';
+import Page from './+page.svelte';
+
+vi.mock('$app/stores', async () => {
+	const { readable } = await import('svelte/store');
+	return {
+		page: readable({ url: new URL('http://localhost/login') })
+	};
+});
+
+vi.mock('$app/forms', () => ({
+	enhance: () => ({ destroy: () => {} })
+}));
+
+describe('login page', () => {
+	it('renders the login form', () => {
+		render(Page);
+		expect(screen.getByText('Willkommen zurück')).toBeInTheDocument();
+	});
+
+	it('renders the brand panel', () => {
+		render(Page);
+		expect(screen.getByText('Mealprep')).toBeInTheDocument();
+	});
+
+	it('sets the page title', () => {
+		render(Page);
+		expect(document.title).toBe('Anmelden — Mealprep');
+	});
+
+	it('does not render any navigation chrome', () => {
+		render(Page);
+		expect(screen.queryByRole('navigation')).not.toBeInTheDocument();
+		expect(screen.queryByText('Planer')).not.toBeInTheDocument();
+		expect(screen.queryByText('Rezepte')).not.toBeInTheDocument();
+	});
+
+	it('renders a link to the signup page', () => {
+		render(Page);
+		const link = screen.getByRole('link', { name: /registrieren/i });
+		expect(link).toHaveAttribute('href', '/signup');
+	});
+});