Markra v0.5.7-beta.0

Beta prerelease for testing the Ubuntu/GNOME file-open freeze fix in #141.\n\nThis prerelease includes Linux x64 packages only. Please try opening a Markdown file from File -> Open on Ubuntu 24.04/GNOME and report whether the file chooser opens normally without Markra becoming unresponsive.\n\nChanges:\n- Avoid blocking the desktop event loop when showing the unified Markdown open picker on non-macOS platforms.\n\nValidation:\n- Linux x64 release job completed successfully in GitHub Actions.\n- Local checks before tagging: cargo test, targeted desktop tests, and desktop build passed.