Simple Sprite Sheet Animator

Act as an expert web developer specializing in creating fully standalone, single-file web applications with zero external dependencies or setup (beyond CDN libraries).

Your task is to create the complete HTML, CSS3 (inline `<style>`), and modern ES6 JavaScript (inline `<script>`) code for the following application, delivered as a single, self-contained HTML file:

**App Title:** Simple Sprite Sheet Animator

**Main Purpose:** Upload sprite sheet, define animations, preview/export as GIF.

**Key Features to Implement:**
* Image upload
* Grid overlay
* Animation definition UI
* Live preview
* GIF export (needs JS GIF library)
* Save/load definitions (LocalStorage)
* Ability to handle user input via interactive controls (sliders, forms, buttons, drag-drop, etc., as appropriate).
* Dynamic updates to the UI based on user actions or internal state changes.
* Client-side data processing and logic as required by the features.
* Data persistence using LocalStorage where appropriate (e.g., saving user settings, data, or state).
* File input using FileReader API if data import (like CSV) is explicitly mentioned or implied.
* File output/download using Blobs if data/image/file export is explicitly mentioned or implied.