feat(#1): Set up monorepo scaffold with pnpm workspaces + TurboRepo

Implements the foundational project structure including:
- pnpm workspaces configuration
- TurboRepo for build orchestration
- NestJS 11.1.12 API (apps/api)
- Next.js 16.1.6 web app (apps/web)
- Shared packages (config, shared, ui)
- TypeScript strict mode configuration
- ESLint + Prettier setup
- Vitest for unit testing (19 passing tests)

Fixes #1

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/app/page.tsx

@@ -0,0 +1,16 @@
+import { Button } from "@mosaic/ui";
+
+export default function Home() {
+  return (
+    <main className="flex min-h-screen flex-col items-center justify-center p-24">
+      <h1 className="text-4xl font-bold mb-8">Mosaic Stack</h1>
+      <p className="text-lg text-gray-600 mb-8">
+        Welcome to the Mosaic Stack monorepo
+      </p>
+      <div className="flex gap-4">
+        <Button variant="primary">Get Started</Button>
+        <Button variant="secondary">Learn More</Button>
+      </div>
+    </main>
+  );
+}