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/themeContext.tsx

@@ -0,0 +1,83 @@
+import React, { createContext, useCallback, useContext, useEffect, useMemo, useState } from 'react'
+
+const STORAGE_KEY = 'zui-theme'
+
+export type Theme = 'light' | 'dark'
+
+function readStored(): Theme | null {
+  try {
+    const s = localStorage.getItem(STORAGE_KEY)
+    if (s === 'light' || s === 'dark') return s
+  } catch (_) {}
+  return null
+}
+
+function systemPrefersDark(): boolean {
+  try {
+    return window.matchMedia('(prefers-color-scheme: dark)').matches
+  } catch (_) {
+    return false
+  }
+}
+
+type ThemeContextValue = {
+  theme: Theme
+  setTheme: (theme: Theme) => void
+  toggleTheme: () => void
+}
+
+const ThemeContext = createContext<ThemeContextValue | null>(null)
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>(() => {
+    const stored = readStored()
+    if (stored) return stored
+    return systemPrefersDark() ? 'dark' : 'light'
+  })
+
+  useEffect(() => {
+    const root = document.documentElement
+    if (theme === 'dark') {
+      root.classList.add('dark')
+    } else {
+      root.classList.remove('dark')
+    }
+  }, [theme])
+
+  useEffect(() => {
+    const m = window.matchMedia('(prefers-color-scheme: dark)')
+    const handler = () => {
+      if (readStored() != null) return
+      setThemeState(m.matches ? 'dark' : 'light')
+    }
+    m.addEventListener('change', handler)
+    return () => m.removeEventListener('change', handler)
+  }, [])
+
+  const setTheme = useCallback((next: Theme) => {
+    setThemeState(next)
+    try {
+      localStorage.setItem(STORAGE_KEY, next)
+    } catch (_) {}
+  }, [])
+
+  const toggleTheme = useCallback(() => {
+    setThemeState((prev) => {
+      const next = prev === 'light' ? 'dark' : 'light'
+      try {
+        localStorage.setItem(STORAGE_KEY, next)
+      } catch (_) {}
+      return next
+    })
+  }, [])
+
+  const value = useMemo(() => ({ theme, setTheme, toggleTheme }), [theme, setTheme, toggleTheme])
+
+  return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>
+}
+
+export function useTheme(): ThemeContextValue {
+  const ctx = useContext(ThemeContext)
+  if (!ctx) throw new Error('useTheme must be used within ThemeProvider')
+  return ctx
+}