build: add Linux targets with stable artifact names

[Fame510]

Summary

Adds an explicit linux build config to electron-builder.yml so npm run build:linux produces stable, version-less artifact names (EasyClaw.AppImage, EasyClaw.snap).

This keeps the landing page's one-click download links durable across releases:

• https://github.com/Fame510/easyclaw/releases/latest/download/EasyClaw.AppImage
• https://github.com/Fame510/easyclaw/releases/latest/download/EasyClaw.snap

Without a fixed artifactName, electron-builder defaults to versioned names (e.g. EasyClaw-1.3.43.AppImage), which would 404 against the stable releases/latest/download/<name> URL after the next version bump. Mirrors how the existing dmg/nsis targets already use fixed names.

linux:
  category: Utility
  artifactName: ${productName}.${ext}
  target:
    - AppImage
    - snap

Testing

• npm run typecheck passes
• npm run lint passes
• Verified npm run build:linux produces EasyClaw.AppImage / EasyClaw.snap and the packaged AppImage launches the wizard on Linux.

Landing page that consumes these URLs (served from gh-pages): https://fame510.github.io/easyclaw/