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

.gitignore

@@ -1,6 +1,13 @@
+# runtime / secrets
 /keys
-/node_modules
-/dist
 /data
 .env
-/*.log
+*.log
+
+# dependencies
+node_modules/
+
+# build outputs
+/dist
+server/dist
+client/dist