Update project structure and enhance frontend functionality

- Added a new AGENTS.md file to document the project structure and conventions.
- Updated .gitignore to include node_modules and refined cursor rules.
- Introduced new backend and frontend components for improved map interactions, including context menus and controls.
- Enhanced API composables for better admin and authentication functionalities.
- Refactored existing components for cleaner code and improved user experience.
- Updated README.md to clarify production asset serving and user setup instructions.

.cursor/rules/frontend-nuxt.mdc

@@ -0,0 +1,11 @@
+---
+description: Nuxt 3 frontend in frontend-nuxt, composables, public API
+globs: "frontend-nuxt/**/*"
+alwaysApply: false
+---
+
+# Frontend (Nuxt 3)
+
+- All frontend source lives in **frontend-nuxt/** (pages, components, composables, layouts, plugins, `public/gfx`). Production build output goes to `frontend/` and is served by the Go backend.
+- Public API to backend: use composables — e.g. `composables/useMapApi.ts`, `useAuth.ts`, `useAdminApi.ts` — not raw fetch in components.
+- Nuxt 3 conventions; ensure dev proxy in `nuxt.config.ts` points at backend when running locally (e.g. 3080 or 8080).