Expose verified build provenance and About dialog

[emsi]

Summary

• inject the exact source revision into native, AppImage, and Flatpak release builds
• verify artifact provenance in the GitHub release workflow
• add a standard GTK About dialog with version, revision, license, and project information

Validation

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• shellcheck on modified release scripts
• Flatpak manifest parse check
• injected revision smoke check
• dependency license check