CI: optimize to macOS + Flatpak only, Linux replay in Flatpak

[fbraz3]

What changed

• Removed Linux gzip and AppImage jobs from CI
• Kept Linux Flatpak build and macOS build as the only CI gates
• Linux replay tests now run inside Flatpak (instead of gzip bundle)
• CI summary now shows only macOS and Linux Flatpak status

Why

• Releases are published as Flatpak (Linux) and macOS app only
• Flatpak already has build and replay validation in CI
• gzip/AppImage are redundant for production release gates

Validation

• Workflow diagnostics are clean
• Replay tests run in Flatpak runtime on Linux
• macOS replay path unchanged