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

web/vitest.config.ts

@@ -0,0 +1,12 @@
+import react from '@vitejs/plugin-react';
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  plugins: [react()],
+  esbuild: { jsx: 'automatic' },
+  test: {
+    environment: 'jsdom',
+    setupFiles: ['./src/test-setup.ts'],
+    include: ['__tests__/**/*.test.{ts,tsx}'],
+  },
+});