This is a single-page web application for building, testing, and managing prompts for Large Language Models (LLMs). It runs entirely in the browser, using localStorage for data persistence, and requires no backend.

• Modular Block Editor: Drag, drop, and reorder blocks (Instruction, Context, etc.) to compose prompts.
• Template Management: Save, load, and delete multiple prompts.
• Import/Export: Save your prompts to a JSON file or import them into another browser.
• Live Monitoring: Real-time character/token counting.
• Responsive Layout: Collapsible sidebar and adaptive panes for mobile and desktop.
• Light/Dark Theme: Switch between themes for user comfort.
• Live Preview: See the assembled plain-text prompt.
• Client-Side Storage: All your work is saved automatically in your browser's localStorage.
• Zero Dependencies (Runtime): Built with pure vanilla JavaScript.

The entire application state is stored in localStorage under the key promptBuilderState. It contains a prompts array, where each prompt is structured as follows:

{
  "id": "prompt_123",
  "name": "My Awesome Prompt",
  "createdAt": "2023-10-27T10:00:00.000Z",
  "updatedAt": "2023-10-28T11:00:00.000Z",
  "blocks": [
    {
      "id": "block_abc",
      "type": "Instruction",
      "content": "Summarize the following text in three bullet points.",
      "isCollapsed": false
    }
  ]
}