Add macOS icon configuration for electron-builder

[Copilot]

Adds dedicated macOS icon asset and configures electron-builder to use it for macOS builds.

Changes

• Created assets/icons/ directory for organized icon assets
• Added macos-icon.png (256×256 PNG) at assets/icons/macos-icon.png
• Configured electron-builder to reference the icon via mac.icon field in package.json

The icon will be automatically applied when building the macOS app with npm run build or npm run build-dmg.

Original prompt

Add the macOS icon provided in the image to the repository. The icon should be named macos-icon.png and placed inside the assets/icons directory. Create the assets/icons directory if it does not exist. Ensure no conflicts with existing files. Reference the macOS icon appropriately if necessary for macOS builds.

This pull request was created from Copilot chat.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.