feat(web): migrate to Next.js App Router and add design tokens (ADR 0018)

The Sprint 1 shell moves to /app with its own root layout so its dark theme does not leak. Figma tokens ported with light and dark parity.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014SdXCbrN5PtGExkpvMaMHx

e2e/tests/chat-flow.test.ts

@@ -1,7 +1,7 @@
 import { test, expect } from '@playwright/test';
 
 test('user signs up and sees the chat interface', async ({ page }) => {
-  await page.goto('/');
+  await page.goto('/app');
 
   await page.getByLabel('Email').fill(`e2e-${Date.now()}@example.com`);
   await page.getByLabel('Password').fill('password123');