Flutter: macOS, Linux, and Windows desktop support

[enzoftware]

Description

Add desktop platform support (macOS, Linux, Windows) to the silvestre Flutter package.

Tasks

• macOS: Configure build to produce .dylib or .a and link via CocoaPods/SPM
• Linux: Configure CMake build to compile Rust and link .so
• Windows: Configure CMake build to compile Rust and link .dll
• Verify flutter_rust_bridge codegen works for all desktop targets
• Test on each desktop platform

Acceptance Criteria

• flutter run -d macos works and can call Rust functions
• flutter run -d linux works and can call Rust functions
• flutter run -d windows works and can call Rust functions
• No platform-specific code in the Dart API layer