Add app icon, refresh button, and packaging config

- Add build/icon.svg (1024x1024 source) and build/icon.png
  (electron-builder picks these up automatically to generate
  the .icns/.ico for each platform)
- Add refresh button to the TopBar — useful when skills are
  installed via the CLI while the app is open
- Add electron-builder config + sharp dev-dep in package.json
  so `npm run pack` / `npm run dist:mac` work
- Ignore release/ (electron-builder output) and notes/
  (personal learning notes)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>