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.

frontend/Dockerfile

@@ -0,0 +1,21 @@
+# Multi-stage: build Vite app, then serve with Nginx (Alpine).
+# Frontend only; backend is a separate service.
+
+# Stage 1: build
+FROM node:20-alpine AS builder
+WORKDIR /app
+
+COPY package.json package-lock.json* ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# Stage 2: serve
+FROM nginx:alpine
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -q -O /dev/null http://localhost:80/ || exit 1
+CMD ["nginx", "-g", "daemon off;"]