docs: added utils.js build script documentation (#1402)

src/gui/doc/utils.md

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+# utils.js — GUI Build Script (Overview)
+
+This file is responsible for:
+- Generating production and development builds of the GUI
+- Merging and minifying JS/CSS files
+- Converting icon files to base64
+- Bundling core GUI logic using Webpack
+- Generating the HTML structure dynamically for development mode
+
+## Main Functions
+
+### 🔧 build(options)
+Runs the full GUI build process.
+
+**Steps it performs:**
+1. Deletes and recreates the `/dist` folder
+2. Merges JavaScript libraries → `dist/libs.js`
+3. Converts all `src/icons/*.svg/png` to base64 → stores in `window.icons`
+4. Merges and minifies CSS → `dist/bundle.min.css`
+5. Uses Webpack to bundle `src/index.js` and dependencies → `dist/main.js`
+6. Prepends `window.gui_env = "prod"` and writes it as `dist/gui.js`
+7. Copies static assets like images, fonts, manifest, etc.
+
+### 🛠️ generateDevHtml(options)
+Dynamically builds the HTML string for development mode.
+
+**What it includes:**
+- Meta tags (SEO + social)
+- CSS & JS includes (based on env)
+- Inline base64 image data
+- JS entry points for dev (`/index.js`) or prod (`/dist/gui.js`)
+
+---
+
+## Related Files
+
+| File             | Role                                 |
+|------------------|---------------------------------------|
+| `build.js`       | Just imports and calls `build()`      |
+| `BaseConfig.cjs` | Provides Webpack config used in build |
+| `static-assets.js` | Lists paths to JS, CSS, icons, etc   |
+
+---
+