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/Dockerfile

@@ -0,0 +1,21 @@
+# Single-stage, Alpine, Node 20. Production backend (no DB).
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Dependencies only
+COPY package.json package-lock.json* ./
+RUN npm ci --omit=dev
+
+COPY src ./src
+
+# Non-root user
+RUN addgroup -g 1001 -S app && adduser -u 1001 -S app -G app
+USER app
+
+EXPOSE 8080
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD node -e "require('http').get('http://localhost:8080/health', (r) => process.exit(r.statusCode === 200 ? 0 : 1))"
+
+CMD ["node", "src/index.js"]