v0.2.0 of velopack_flutter

A flutter implementation of Velopack to enable easy distribution and auto-updates for flutter apps Changelog excerpt: - Upgrade Rust Velopack crate to 1.2.0 - Upgrade FRB to 2.12.0 - Added Dart types mirroring for UpdateInfo - Initialize Velopack with URL just once after init - Added Dart initializeVelopack helper for one-step setup - Raised minimum Dart SDK to 3.6.0 for build hook support - Added download progress stream sink to Flutter - Used build hooks instead of CargoKit - Better example UI