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/api/src/app.controller.ts

@@ -0,0 +1,29 @@
+import { Controller, Get } from "@nestjs/common";
+import { AppService } from "./app.service";
+import type { ApiResponse } from "@mosaic/shared";
+
+interface HealthStatus {
+  status: string;
+  timestamp: string;
+}
+
+@Controller()
+export class AppController {
+  constructor(private readonly appService: AppService) {}
+
+  @Get()
+  getHello(): string {
+    return this.appService.getHello();
+  }
+
+  @Get("health")
+  getHealth(): ApiResponse<HealthStatus> {
+    return {
+      success: true,
+      data: {
+        status: "healthy",
+        timestamp: new Date().toISOString(),
+      },
+    };
+  }
+}