chore(repo): restructure as monorepo with server and client workspaces

- move NestJS app into server/ subdirectory
- add client/ React+TypeScript (Vite) app with Hello World
- update docker-compose to build and run both services
- add root package.json declaring npm workspaces
- update .gitignore to cover node_modules and dist at all depths

server/eslint.config.mjs

@@ -0,0 +1,8 @@
+import tseslint from 'typescript-eslint';
+import eslintConfigPrettier from 'eslint-config-prettier';
+
+export default tseslint.config(
+  { ignores: ['dist/**', 'node_modules/**'] },
+  ...tseslint.configs.recommended,
+  eslintConfigPrettier,
+);