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/app/(legacy)/app/layout.tsx

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+import type { ReactNode } from 'react';
+import '../../../src/styles/global.css';
+
+export const metadata = { title: 'Wursor' };
+
+// Interim product shell from Sprint 1. Its own root layout keeps its dark theme
+// from leaking into the marketing site. Replaced by the designed screens.
+export default function LegacyLayout({ children }: { children: ReactNode }) {
+  return (
+    <html lang="en">
+      <head>
+        <link rel="preconnect" href="https://fonts.googleapis.com" />
+        <link rel="preconnect" href="https://fonts.gstatic.com" crossOrigin="" />
+        <link
+          href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700;800;900&family=JetBrains+Mono:wght@400;500&display=swap"
+          rel="stylesheet"
+        />
+      </head>
+      <body>{children}</body>
+    </html>
+  );
+}

web/app/(legacy)/app/page.tsx

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+import { LegacyApp } from '../../../src/LegacyApp.tsx';
+
+export default function Page() {
+  return <LegacyApp />;
+}

web/app/(marketing)/layout.tsx

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+import { GeistMono } from 'geist/font/mono';
+import { GeistSans } from 'geist/font/sans';
+import type { Metadata } from 'next';
+import type { ReactNode } from 'react';
+import '../../src/styles/tokens.css';
+import '../../src/styles/marketing.css';
+
+export const metadata: Metadata = {
+  title: { default: 'Wursor', template: '%s · Wursor' },
+  description: 'Your website, handled. Tell Wursor what to change. Watch it work. Approve what you like.',
+};
+
+export default function MarketingLayout({ children }: { children: ReactNode }) {
+  return (
+    <html lang="en" className={`${GeistSans.variable} ${GeistMono.variable}`}>
+      <body>{children}</body>
+    </html>
+  );
+}

web/app/(marketing)/page.tsx

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+export default function HomePage() {
+  return <main>Wursor</main>;
+}