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/src/lib/registerBuiltinNodes.tsx

@@ -0,0 +1,93 @@
+/**
+ * Registers built-in node types (config, render, variable, function).
+ * Import this once at app startup (e.g. in main.tsx) so the registry is populated.
+ */
+
+import React from 'react'
+import { ScrollText, Sparkles, Variable, Code2 } from 'lucide-react'
+import { registerNodeType } from './nodeRegistry'
+import { NODE_HELP } from './nodeHelp'
+import ConfigNode from '../components/nodes/ConfigNode'
+import RenderingNode from '../components/nodes/RenderingNode'
+import VariableNode from '../components/nodes/VariableNode'
+import FunctionNode from '../components/nodes/FunctionNode'
+
+const ICON_CLASS = 'mr-2 h-4 w-4'
+
+export function registerBuiltinNodes(): void {
+  registerNodeType({
+    id: 'config',
+    component: ConfigNode,
+    defaultStyle: { width: 320, height: 320 },
+    defaultData: { configType: 'plantuml', content: '@startuml\n\n@enduml\n', title: '' },
+    idPrefix: 'cfg_',
+    hasInput: true,
+    hasOutput: true,
+    allowedSourceTypes: ['config', 'variable', 'function'],
+    allowedTargetTypes: ['config', 'render'],
+    help: NODE_HELP.config,
+    menuLabel: 'Config',
+    menuIcon: <ScrollText className={ICON_CLASS} />,
+    getDefaultData: (newId) => ({
+      configType: 'plantuml',
+      content: '@startuml\n\n@enduml\n',
+      title: newId ?? '',
+    }),
+    getResetData: (nodeId) => ({
+      configType: 'plantuml',
+      content: '@startuml\n\n@enduml\n',
+      title: nodeId ?? '',
+    }),
+    connectionLabel: 'adding input',
+  })
+
+  registerNodeType({
+    id: 'render',
+    component: RenderingNode,
+    defaultStyle: { width: 384, height: 320 },
+    defaultData: { viewportWidth: 1200, viewportHeight: 800 },
+    idPrefix: 'rnd_',
+    hasInput: true,
+    hasOutput: false,
+    allowedSourceTypes: ['config'],
+    help: NODE_HELP.render,
+    menuLabel: 'Renderer',
+    menuIcon: <Sparkles className={ICON_CLASS} />,
+    connectionLabel: 'rendering',
+  })
+
+  registerNodeType({
+    id: 'variable',
+    component: VariableNode,
+    defaultStyle: { width: 224, height: 180 },
+    defaultData: { value: '', valueType: 'string' },
+    idPrefix: 'var_',
+    hasInput: false,
+    hasOutput: true,
+    allowedTargetTypes: ['config', 'function'],
+    help: NODE_HELP.variable,
+    menuLabel: 'Variable',
+    menuIcon: <Variable className={ICON_CLASS} />,
+  })
+
+  registerNodeType({
+    id: 'function',
+    component: FunctionNode,
+    defaultStyle: { width: 288, height: 260 },
+    defaultData: {
+      body: `function(num, kwargs) {
+  return num + (kwargs.bar || 0);
+}`,
+    },
+    idPrefix: 'fn_',
+    hasInput: true,
+    hasOutput: true,
+    allowedSourceTypes: ['config', 'variable', 'function'],
+    allowedTargetTypes: ['config', 'function'],
+    help: NODE_HELP.function,
+    menuLabel: 'Function',
+    menuIcon: <Code2 className={ICON_CLASS} />,
+    getResetData: () => ({ body: '' }),
+    connectionLabel: 'adding input',
+  })
+}