feat: add node help system and registry for extensible node types

- Implemented a help system for different node types (config, render, variable, function) with detailed usage instructions.
- Created a node registry to manage node types, including registration, retrieval, and validation of connections between nodes.
- Defined central node and edge types for the application to streamline state management.
- Added Nunjucks autocomplete functionality to enhance user experience in template editing.
- Developed a syntax highlighting parser for PlantUML and Nunjucks within the CodeMirror editor.
- Registered built-in node types at application startup, including their default configurations and help entries.
- Introduced a theme context provider to manage light/dark mode preferences across the application.
- Created utility functions for class name management using clsx and tailwind-merge.
- Set up Tailwind CSS for styling with custom themes and responsive design.
- Configured Vite for development with proxy settings for backend API calls and Kroki diagram service.

backend/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "zui-backend",
+  "version": "1.0.0",
+  "private": true,
+  "description": "Minimal Express API for Zui (todos CRUD, no DB)",
+  "main": "src/index.js",
+  "scripts": {
+    "start": "node src/index.js",
+    "dev": "node --watch src/index.js"
+  },
+  "engines": {
+    "node": ">=20"
+  },
+  "dependencies": {
+    "cors": "^2.8.5",
+    "express": "^4.21.0"
+  }
+}