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>

tsconfig.json

@@ -0,0 +1,18 @@
+{
+  "extends": "./packages/config/typescript/base.json",
+  "compilerOptions": {
+    "baseUrl": ".",
+    "paths": {
+      "@mosaic/shared/*": ["packages/shared/src/*"],
+      "@mosaic/ui/*": ["packages/ui/src/*"],
+      "@mosaic/config/*": ["packages/config/*"]
+    }
+  },
+  "references": [
+    { "path": "packages/shared" },
+    { "path": "packages/ui" },
+    { "path": "apps/api" },
+    { "path": "apps/web" }
+  ],
+  "exclude": ["node_modules", "**/node_modules", "dist", ".next"]
+}