feat(auth): add BrandPanel component for signup screen

Renders brand identity with logo, app name, tagline, and feature icons
on green-dark background. Responsive: banner on mobile, 440px column
on desktop.

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

frontend/src/lib/auth/BrandPanel.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/svelte';
+import BrandPanel from './BrandPanel.svelte';
+
+describe('BrandPanel', () => {
+	it('renders the app name', () => {
+		render(BrandPanel);
+		expect(screen.getByText('Mealprep')).toBeInTheDocument();
+	});
+
+	it('renders the tagline', () => {
+		render(BrandPanel);
+		expect(screen.getByText('Plan meals, eat well, waste less')).toBeInTheDocument();
+	});
+
+	it('renders the logo emoji', () => {
+		render(BrandPanel);
+		expect(screen.getByText('🥗')).toBeInTheDocument();
+	});
+
+	it('renders three feature icons with labels', () => {
+		render(BrandPanel);
+		expect(screen.getByText('Plan')).toBeInTheDocument();
+		expect(screen.getByText('Cook')).toBeInTheDocument();
+		expect(screen.getByText('Shop')).toBeInTheDocument();
+	});
+
+	it('renders feature emojis', () => {
+		render(BrandPanel);
+		expect(screen.getByText('📅')).toBeInTheDocument();
+		expect(screen.getByText('🍳')).toBeInTheDocument();
+		expect(screen.getByText('🛒')).toBeInTheDocument();
+	});
+});