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

client/Dockerfile

@@ -0,0 +1,19 @@
+# ── Build stage ───────────────────────────────────────────────────────────────
+FROM node:22-slim AS builder
+
+WORKDIR /app
+
+# Copy root lockfile + workspace manifests so npm ci can resolve the full graph
+COPY package*.json ./
+COPY client/package.json ./client/
+RUN npm ci -w client
+
+COPY client ./client
+RUN npm run -w client build
+
+# ── Runtime stage ─────────────────────────────────────────────────────────────
+FROM nginx:alpine AS runtime
+
+COPY --from=builder /app/client/dist /usr/share/nginx/html
+
+EXPOSE 80