feat: app launcher icon generator (iOS AppIcon + Android mipmap)

- Add devDependency sharp@0.34.2 and npm run gen:app-icon
- scripts/generate-app-icon.cjs: source app-icon.png or bootsplash logo, #111827 flatten
- Populate AppIcon.appiconset PNGs + Contents.json filenames
- Regenerate ic_launcher / ic_launcher_round per density
- Document in development.md and README quick start

Made-with: Cursor

Author: maximcoding

README.md

@@ -49,7 +49,9 @@ cd react-native-starter
 npm install
 npx pod-install ios
 cp .env.example .env
-````
+```
+
+**Launcher icon (optional):** add `assets/app-icon.png` (1024×1024, square), or use the default generator source (`assets/bootsplash/logo@4x.png`), then run `npm run gen:app-icon`. See [docs/development.md](docs/development.md#assets--guards).
 
 Edit `.env` as needed, then: