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/test/__mocks__/jsdom.ts

@@ -0,0 +1,18 @@
+const mockElement = {
+  outerHTML: '<div id="mock">mock content</div>',
+  textContent: "mock content",
+};
+
+export class JSDOM {
+  constructor(
+    public html: string,
+    public options?: Record<string, unknown>,
+  ) {}
+  get window() {
+    return {
+      document: {
+        querySelector: () => mockElement,
+      },
+    };
+  }
+}