Ensure Linux AppImage artifacts are executable

[QuickMythril]

Adds a small post-build helper for Linux AppImage artifacts.

electron-builder can produce the final .AppImage file without the executable bit set, which means users may need to run chmod +x manually before launching it.

This adds a helper that scans electron/dist for generated .AppImage files and applies 0755 permissions after Linux packaging completes.

The important pieces are:

• add electron/scripts/chmod-linux-appimages.mjs
• run it after Linux AppImage builds complete
• keep the permission change scoped to final .AppImage artifacts

This is intended as a small packaging fix. If applying the same idea to a newer Qortal Hub branch, the helper can stay the same and only the package-script wiring may need to match that branch's current build commands.

Validation:

• parsed electron/package.json with Node
• ran npm --prefix electron run chmod:linux:appimages against a dummy electron/dist/Test.AppImage and confirmed it changed from 0644 to 0755
• ran git diff --check