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.

.gitignore

@@ -1,5 +1,6 @@
 # dependencies
-/node_modules
+node_modules
+
 /.pnp
 .pnp.js
 
@@ -9,14 +10,15 @@
 # production build
 /build
 /dist
+frontend/dist
+
+# environment (keep .env.example if you add one)
+.env
+.env.*
+!.env.example
 
 # misc
 .DS_Store
-.env
-.env.local
-.env.development.local
-.env.test.local
-.env.production.local
 
 # logs
 npm-debug.log*
@@ -38,3 +40,7 @@ Thumbs.db
 .cache
 .parcel-cache
 
+# Docker (optional local overrides)
+docker-compose.override.yml
+docker-compose.override.yaml
+