v3.0.0-beta.1

First public v3 beta. Desktop contract: Windows 10/11 amd64/arm64, macOS Intel/Apple Silicon, Linux amd64/arm64 with GTK4 + WebKitGTK 6.0 by default and GTK3 legacy through v3.0.x. Go 1.25+. Mobile remains experimental. Cut from releases/v3-beta after Beta candidate gates pass.