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/flowContext.tsx

@@ -0,0 +1,29 @@
+import React from 'react'
+import type { Connection } from '@xyflow/react'
+import type { AppNode, AppEdge } from '@/lib/nodeTypes'
+
+export type ConnectionFrom = { nodeId: string; sourceHandle?: string } | null
+
+export type FlowActions = {
+    pasteAtViewportCenter: () => void
+    fitView: () => void
+}
+
+export type FlowContextValue = {
+    nodes: AppNode[]
+    setNodes: (updater: AppNode[] | ((prev: AppNode[]) => AppNode[])) => void
+    edges: AppEdge[]
+    setEdges: (updater: AppEdge[] | ((prev: AppEdge[]) => AppEdge[])) => void
+    renamingNodeId: string | null
+    setRenamingNodeId: (id: string | null) => void
+    /** Set when user starts dragging from an output handle; cleared on connect end. Used to highlight valid targets. */
+    connectionFrom: ConnectionFrom
+    setConnectionFrom: (v: ConnectionFrom) => void
+    isValidConnection: (connection: Connection) => boolean
+    /** Set by FlowKeyboardShortcuts so Node menubar can trigger paste / fit view. */
+    flowActionsRef: React.MutableRefObject<FlowActions | null>
+}
+
+const FlowContext = React.createContext<FlowContextValue | null>(null)
+
+export default FlowContext