feat: add WAV file transfer and golden fixture

[matveyb9]

Summary

• adds a thin file_picker adapter for user-selected WAV open/save dialogs
• keeps Rust as the only authority for ADLP/WAV validation: imported bytes go through decodeWav before the UI exposes metadata
• adds export and import actions to the Flutter workbench, with injectable adapters and widget coverage
• commits a deterministic ADLP v1 WAV golden fixture with documented generation input, size, SHA-256 and byte-for-byte Rust regression coverage
• updates English/Russian documentation and the GitHub Pages status without claiming live-audio support

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test
• flutter build web --release
• flutter build linux --debug

Scope boundary

The app now saves and selects local WAV files. It does not yet play audio, capture a microphone, route cable/Bluetooth audio, authenticate a callsign, or encrypt a transfer.