feat: add release build script

- scripts/build-release.sh: builds, signs, and copies .app to dist/release/
- Bundle targets narrowed to app + dmg
- npm run release convenience script
- README documents the release build workflow

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -33,6 +33,16 @@ npm run tauri dev
 npm run tauri build
 ```
 
+## Release Build
+
+```bash
+npm run release
+```
+
+This runs the full Tauri production build, ad-hoc code signs the `.app` bundle, and copies the output to `dist/release/`.
+
+The resulting `GUI Video Clipper.app` can be launched directly or dragged to `/Applications`.
+
 ## Architecture
 
 GUI Video Clipper is built with [Tauri v2](https://v2.tauri.app/) (Rust backend) and [Svelte 5](https://svelte.dev/) (TypeScript frontend).